I'm going to make this post here as a last-ditch effort to bring Grass/Dragon back into the good graces of the CAP community. I understand that both Jas and Deck, along with a handful of others, have serious reservations about it; I am not ignoring their legitimate concerns but rather trying to address them in the context of this typing's advantages. I do however have a lot to say in support of this typing, so this is going to be a long post (sorry).
How does this typing address the biggest weaknesses of sun teams?
The first and probably biggest obstacle for Sun teams is opposing weather, especially Rain. More to the point, the standard Ninetales/Venusaur/Dugtrio/Heatran/Forretress/Volcarona teammates have enormous trouble reacting to Politoed switch-ins, and they consistently struggle at any given moment that Drizzle is active, with the vast majority of Sunmons unable to switch into rain offense. Venusaur is the only Sun staple not weak to Water (Forry has crap SpD), so it can't tank those Rain-boosted-or-otherwise Water attacks all on its own, waiting for a chance to wedge Ninetales back into battle. Because of this extreme weakness, Sun has found itself to be the only perma-weather playstyle forced to waste moveslots on it's own weather-summoning move, Sunny Day. Jas and others have argued that a Water resistance isn't necessary since Sun is the chosen playstyle here, but I really think that line of reasoning fails to account for Sun's poor ability to get Sun back, most especially when facing Drizzle teams.
Venusaur is also Sun's most common Electric resist and best answer to Rotom-W, putting more pivoting and defensive pressure on the team's primary sweeper in the face of opposing rain (Rotom-W sees very diverse usage on a lot of different play styles), which hurts Sun's viability a lot. For this reason, supplementary teammates to the standard Sun build are tightly constrained to Water and Electric resists specifically, preferably with a Ground resist/immunity. Sun frequently calls on Sunny Day shenanigans and Dragons like Latios to back Venusaur up when it comes to avoiding Water-type assaults and pivoting around opposing weather starters, a role Grass/Dragon can fill much more thoroughly.
A Dragon/Grass teammate's 4x resistances to both Water and Electric would allow it to be a better pivot vs. rain-boosted attacks and Rotom-W than Latios and Venusaur are, taking enormous defensive pressure off Venusaur itself. Unlike other Grass typings suggested, the Dragon pairing takes pressure off of CAP's statistical bulk when it comes to tanking powerful Hydro Pumps, especially when boosted by the rain, multiple times in a match. Grass STAB in return threatens Politoed and the other Water-types off the field (even Gastrodon) better than Lati@s, and even crosses over into Sand-checking territory. Dragon STAB keeps the Dragons that threaten Sun from switching in so easily and often (Scarfed or naturally fast CAPs could realistically help with Sun's Lati@s problem via a combination of deterrence and possibly revenge-killing). Dragon STAB also draws in defensive Steels, which Sun teams can take good advantage of, especially Ninetales if opposing weather is up. At the same time, the typing's 4x weakness to Ice, more glaring than even its Dragon weakness (Heatrans and Forretresses take those hits), is an easy target for weak moves like non-STAB HP-Ice or Ice Beam, which Ninetales (and just about anything else on a Sun team) can switch in on to take minimal damage, reset Sun, or set up. A smart player running Ninetales + CAP5 could even find themselves able to switch Ninetales directly into Polioed to take an Ice Beam aimed at the more obvious CAP5 switch-in! In terms of roles, this typing plays the anti-Rain pivot better than any other. An additional Ground resistance is huge for Sun teams; Ninetales, Heatran, Venusaur, and Volcarona all hate Ground moves, Dugtrio can't tank anything, and Forretress lacks recovery enough to not be worn down by neutral blows, leaving little space in team building to address the popular attacking type (in addition to Water/Electric resists). None of the other supported typings boast this Ground resistance, and the Electric types only exacerbate this problem for Sun teams. Grass/Dragon would address Electric/Ground/Water all at once with targeted defensive matchups and useful offensive presence. In return, common Sunmons can take U-turns, Dragon moves, and especially Ice moves for CAP5, no problem. Heatran, already a staple on Sun teams, resists everything Grass/Dragon is weak to, and except for Fighting-type attacks, the reverse is also true.
A Grass/Dragon has everything to gain from the Sun offensively. Fire coverage, even via Hidden Power, in conjunction with Grass and Dragon STABs, makes for an incredible coverage set that only misses out on Heatran (for Dugtrio?) and hits most common Fighting, Water, and Steel-types for super-effective damage. In Sun, CAP gets a 1.5x power bonus on all three attack moves and so is most effective in that weather. If we are careful with coverage, we can actually make CAP5 wallable by certain Steel-types this way if rain is up, which would make CAP a much more effective Poke in Sun than in Rain (where Ferrothorn outclasses as a Grass-type). Grass/Dragon/HP-Ground too would be a viable option as a Heatran lure for Sun teams that have trouble with it, and retains good overall coverage. Recovery moves like Synthesis are also available to Grass-types for use in the Sun.
In any case, the synergy Grass/Dragon offers to Sun teams as an anti-rain pivot, by typing alone, is phenomenal, and it gives us a much better head start into achieving our desired direction. It patches up big holes in Sun strategy with its targeted resistances and offensive STABs while common Sunmons can take U-turns, Dragon moves, and Ice moves for CAP5.
What slot does it fill in a sun team that allows the team the flexibility commonly available to rain and sand?
Rapid Spin could be neat here, for two reasons. The first reason is that an offensive spinner would do Sun a much bigger favor than its current options, as it can cause more switches in more situations than Forretress and Donphan. On top of that, the CAP would be able to take on Jellicent with incredible ease, 4x resisting its typically mono-Water attack set and threatening with Grass STAB, while Gengar would have trouble switching into Dragon attacks and has only the incredibly rare HP-Ice to hit it with (or Sludge Bomb, but come on), so it keeps OU's spin-blockers at bay. The second reason is that it would take the place of Forretress, who is such a momentum-killer for the primarily fast and offensive Sun teams and cannot double as a check to opposing weather. With Forry's primary function covered, Sun teams can look to a wider variety of teammates that don't absolutely need access to Rapid Spin. Mainly, though, where Sun is pretty crappy at maintaining its weather effects against rain, this typing would diminish the desire for near-gimmicks like Weather Trapper Heatran and Sunny Day Ninetales/Venusaur and open up a couple moveslots around the team by making it easier for Ninetales to switch in. Things like Gliscor, Mamoswine, or additional Sun sweepers could fill in gaps much better for the Sun play style with Forretress and Donphan brushed aside, while Sunmons as a group would appreciate the lack of entry hazards. This CAP could fit in Venusaur's slot at the same time and tweak it into an offensive pivot, synergizing well with something like Volcarona as the primary sweeper and opening up a teamslot. Entry hazards could work along the same lines and are a great tactic for Pokemon designed to cause switches.
As an aside, I am not convinced that Pursuit trapping fills the role of Trapper here, and definitely not as a replacement to Dugtrio. Dugtrio is already on Sun teams for the sole reason of removing Heatran; I don't see how adding another teammate to specifically trap an additional specific threat (just Lati@s) helps at all, especially considering you would need either a physical Ice move, good mixed attacking stats with HP-Ice to cover the other Dragons, and/or SpD enough to take repeated Draco Meteors, at which point you're just looking at tons of stats. At the very least, Pursuit as the only justification for Dark typing is kind of weak and doesn't appear to relate to the concept in any reasonable way.
What gaps in the typing are potential issues that could be filled by an existing Pokemon that could be introduced into one of the slots that are opened up?
Offensively, Grass/Dragon doesn't need much help on Sun teams that already trouble Steels, so it'll fit right in. Filling a slot on a Sun team means the strategy won't lean so heavily on other Dragons to patch up its weaknesses in those last couple team slots, invariably allowing a greater variety of strategies. Mamoswine would probably see an uptick in usage on Sun; with Thick Fat it resists Ice, and it's Ground/Ice STABs do a good job of targeting opposing Dragons, Steels, and Sun teams. Ice and Bug resists are easy to come by, Steels will be around to absorb Dragon attacks until the end of time, and Flying is much less of a concern now that Tornadus-T is banned, while Sun lowers Hurricane's accuracy to begin with. I can't say for sure which Pokemon will be used to fill in the gaps, but this typing fits well enough on current Sun teams that the variety of viable candidates will be large.
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Okay now I'm going to make this post even longer because I apparently have a lot to defend here regarding the Dragon typing and I'll just be beating a dead horse if I try to stretch this out into multiple posts. Firstly, I would like to point out that this typing's 4x Water resist let's us be more creative during our ability stages; we could pursue pro-Sun abilities or power adjusting abilities or really anything not named Water Absorb/Storm Drain, the two of which run a big risk of stifling our ability discussions if we don't address Water resistance here. Same thing goes for Sun's Ground-type weakness and the popularity of Electric typing in this thread. On the flip side, no abilities are going to stop Dragon spam, leaving bulky stats as the only method for switching into Dragon attacks anyway; we must either be set on high defensive stats (polljumping) or content to not try and switch into Dragons much at all (this would suggest that being Dragon-weak actually isn't a real concern for the concept). Being Dragon-weak is actually not much worse than being neutral to Dragon in this metagame; not much can switch into those attacks on either side of the spectrum. Grass/Dragon at least takes neutral-or-better damage from every common coverage move found on OU's Dragons (Kyurem-B being the exception for its Ice STAB), which none of Grass/Electric, Dark/Electric, Grass/Steel, or Steel/Fire can say for themselves.
I am not proposing a Dragon to beat all other Dragons; that is a mischaracterization of the advantages of this typing. Grass/Dragon is for pivoting through common Drizzle tactics for the benefit of resetting Sun and for covering common weaknesses on standard Sun teams. It is also for pairing best with Fire coverage offensively, which is boosted by the Sun. The fact that it can hit every Dragon in OU is a plus, considering the concept, but it isn't really the selling point of this typing at all. It is true that Dragons can find themselves welcome on any type of team, and Grass/Dragon would get a pseudo-Water resist from Rain and be able to absorb Electric and Grass attacks aimed at Water-types. However, considering this practically (in the absence of legendary stats and Water-type attacks), this Poke can only be outclassed by Lati@s and Ferrothorn when competing for teamslots in rain. It will most likely be walled by every Steel-type in rain, and it's also weak to Hurricane and U-turn, which rain teams can't afford. On Sun teams, this just isn't the case; in fact, the opposite is true, that Sun currently has nothing of note dedicated to reacting to Rain beyond running backup Sunny Days on one or more teammates, forcing Venusaur to double-dip tactically, or just running Dragons in one or two of its few available teamslots anyway. The way the two weather styles are currently constructed, Grass/Dragon may be usable in rain, but it is BETTER in Sun, or more importantly not outclassed in what it's trying to do.
Okay, moving on to the Ice weakness: I don't get what the fuss is about. Politoed, Starmie, Vaporeon, some Tentacruel, and some Keldeo carry Ice moves amongst the various common Watermons, this is true. Jolteon and Thundurus-T commonly run HP-Ice for BoltBeam coverage. Does that mean those Pokes will be launching Ice Beams every time they enter the field? If yes, then GOOD. Sun Pokes can switch into that stuff all day, and Ninetales can take their Water-power away in the process. Grass-STAB and any kind of offensive presence will prevent CAP from being a sitting duck against these Water-types; the ability to force them out for fear of being KOed is the very definition of what a pivot is supposed to do. I've actually explained this now 3 times in 2 posts, so I'll just leave it be.
Well I hope you read all that; as you can see there is plenty justification for Grass/Dragon typing as it pertains to Sun strategy and as it pertains to the rest of our process. If I am called to it, I will further this argument, but in the meantime I will just sit here scratching my head as to why everyone wrote it off so immediately and picked up a bandwagon on some really questionable typings (Dark/Electric for Sun teams? Grass/Electric for Sun teams?).