I love the logic behind Birkal's defense of Dragon/Steel. I especially appreciate his point that dragon's stab is great for bellydrum, precisely because we can't make it CAP6 a better choicebander than Ky-B or Haxorus, and that bellydrum lets you use dragon claw over outrage! I also agree that we don't want a pokemon weak to volt/turn. I also agree that this typing allows multiple 4th moves, which I think is crucial. That said my only concern with dragon/steel is that it is so good defensively I worry it'll be hard not to use it as a defensive mon.
The way I see it, the risk on typing this CAP is as follows - if its too good defensively it risks a pokemon that will be used defensively and not as a bellydrummer, and if its too good offensively then it risks being used as a revenge killer, or (if its STAB is boosted by rain/sun) a weather-mon (otherwise water/electric would be kinda cool).
One type I want to explore more is Dragon/Fighting, which Birkal and I both brought up sorta by the way - this type resists water, electric, grass, fire, steel, rock, bug. That means you resist volt/turn! We have access to stab priority if we want to include it in the movepool stage. We have some bad weaknesses (psychic, flying, dragon, ice) but not tons of them. We resist stealth rock. also with bellydrum lower power fighting moves like DRAIN PUNCH(!), Brick break, force palm, and low sweep become more viable, and means we don't have to give it close combat or even superpower. (I hope that wasn't poll-jumping). My fear for dragon/fighting is basically the opposite as my fear for dragon/steel, its so offensively good it risks not needing bellydrum. But I think keeping its movepool and stats in check will help with that. As I see, dragon/fighting is better than dragon/water, another type some people like, in lots of ways. You won't have to worry abt weather affects very much. It has better resists and is better offensively. That said dragob/water has 2 quad-resists, which is really nice for this CAP and neither dragon/steel or dragon/fighting match that particular advantage to dragon/water. (poison immunity does not matter except for toxic).
Finally, three(ish) other kind of odd-ball types to consider:
Ground/Water - I would love to have this thing be a ground type, precisely because it would be awesome to not get twaved. But ground has many bad weaknesses. I think water/ground at least lets us imagine a pokemon with an immunity (something reachzero wants), only 1 weakness, a few good resists (including to stealth rock). Offensively the stabs combo well. That said, I don't like the idea of a CAP6 that is affected much by weather...
Flying/Electric - Not weak to voltswitch, or u-turn. A flying electric would be pretty anti-meta, and there are (barely) decent enough physical stabs on both sides to imagine using (drill peck/ thunderpunch). Rock and ice weakness is bad, but hey you're not weak to spikes/tspikes and you have nifty resists to fighting and steel and bug and is immune to ground. Offensively you have pretty good coverage with just your stabs... I kinda like this type actually!
Rock/Ghost or Rock/Fighting or Rock/Flying (with volt absorb! j/k)... yeah probably not. I guess there are reasons there are only 2 rock-types in OU. One problem I have with ghost typing is that you end up making a pokemon people will want to use as a defensive spin-blocker and rock/fighting is like ok i'll just use terrakion. But I still think a rock-type bellydrummer would be amazing because hello rock slide instead of stonemiss But Rock might just suck too much. I wish Rock/Grass wasn't weak to steal, bug and fighting, ugh!
The way I see it, the risk on typing this CAP is as follows - if its too good defensively it risks a pokemon that will be used defensively and not as a bellydrummer, and if its too good offensively then it risks being used as a revenge killer, or (if its STAB is boosted by rain/sun) a weather-mon (otherwise water/electric would be kinda cool).
One type I want to explore more is Dragon/Fighting, which Birkal and I both brought up sorta by the way - this type resists water, electric, grass, fire, steel, rock, bug. That means you resist volt/turn! We have access to stab priority if we want to include it in the movepool stage. We have some bad weaknesses (psychic, flying, dragon, ice) but not tons of them. We resist stealth rock. also with bellydrum lower power fighting moves like DRAIN PUNCH(!), Brick break, force palm, and low sweep become more viable, and means we don't have to give it close combat or even superpower. (I hope that wasn't poll-jumping). My fear for dragon/fighting is basically the opposite as my fear for dragon/steel, its so offensively good it risks not needing bellydrum. But I think keeping its movepool and stats in check will help with that. As I see, dragon/fighting is better than dragon/water, another type some people like, in lots of ways. You won't have to worry abt weather affects very much. It has better resists and is better offensively. That said dragob/water has 2 quad-resists, which is really nice for this CAP and neither dragon/steel or dragon/fighting match that particular advantage to dragon/water. (poison immunity does not matter except for toxic).
Finally, three(ish) other kind of odd-ball types to consider:
Ground/Water - I would love to have this thing be a ground type, precisely because it would be awesome to not get twaved. But ground has many bad weaknesses. I think water/ground at least lets us imagine a pokemon with an immunity (something reachzero wants), only 1 weakness, a few good resists (including to stealth rock). Offensively the stabs combo well. That said, I don't like the idea of a CAP6 that is affected much by weather...
Flying/Electric - Not weak to voltswitch, or u-turn. A flying electric would be pretty anti-meta, and there are (barely) decent enough physical stabs on both sides to imagine using (drill peck/ thunderpunch). Rock and ice weakness is bad, but hey you're not weak to spikes/tspikes and you have nifty resists to fighting and steel and bug and is immune to ground. Offensively you have pretty good coverage with just your stabs... I kinda like this type actually!
Rock/Ghost or Rock/Fighting or Rock/Flying (with volt absorb! j/k)... yeah probably not. I guess there are reasons there are only 2 rock-types in OU. One problem I have with ghost typing is that you end up making a pokemon people will want to use as a defensive spin-blocker and rock/fighting is like ok i'll just use terrakion. But I still think a rock-type bellydrummer would be amazing because hello rock slide instead of stonemiss But Rock might just suck too much. I wish Rock/Grass wasn't weak to steal, bug and fighting, ugh!