Nice post
Jin White! Answers to those two pokemon are imo at the heart of every teambuilding process for rain. It depends a lot on what type of team structure you're seeking, but given the 'they don't necessarily need to switch in' part of your post I'm guessing you lean more toward hyper/heavy offense, which imo is the strongest rain style. Definitely consider
s_aman's Tornadus-T suggestion. LO + Regenerator + Hurricane + U-turn makes it a nightmare for Ferro and Venusaur, even against teams that pack 'counters' to it since it's so fucking fast it can almost always safely U-turn into Kingdra/Tops/Star and let you start laying waste. It makes a great switch-in to Chansey, Ferro and usually Venu (doesn't like taking sludge bomb, but you get a free Hurricane + U-turn so you gain a lot back regardless), while conveniently also checking Keldeo, Scizor, Tyranitar and (with Taunt+Turn) gives you set-up chances against many defensive mons. I recommend Taunt or 4-Attacks (Focus Blast | Superpower and Knock Off) highly over Rain Dance, since with Deo gone IMHO there really isn't ANY pokemon worth using as a secondary setter. It can definitely prove useful, but Toed has some surprising utility & durability on its own and it's usually better long term for you to just be able to keep it alive and double switching around rather than compromise the item and utility of another mon (whom you may end up saccing in the process) for a more limited rain dance. Torn-T also makes a half-decent lead against Sashloom, since you can U-turn off its sash and choose a sleeper, while Breloom has a field day of making you guess/play safe against Politoed leads.
The core I use (see previous page) of Celebi + Torn-T pressures Venusaur, Ferrothorn (free rocks for Celebi) and Breloom very well, and while a well-played Ferro can be troublesome it can't actually switch into Omastar, so it usually eventually ends up getting broken. UNFORTUNATELY this opens you up somewhat to Thundurus-I (Celebi handles all right, but Knock off, NP HP Ice or LO HP Fly = owned Celebi, and all those probably make up +50% of Thundy in higher play). Haven't found the answer for this yet aside from hoping to net a SD with Tops or trying to push it around with rocks.
Slightly unrelated, but I REALLY wish Mamo was as useful on HO rain as the SMOG article made it sound lol. It's a great SR lead for sure, and smacks around Breloom and the genies just fine, but it gets owned just as hard as other rain members by Ferro/Venu/Rotom, so idgi
As for some of your other considered mons:
-Keldeo is obviously fine and great on Rain (don't use with 2 SSers IMO), especially as it eases its using ebelt/LO or even Scarf if you want. But if you ever manage to hit a Ferrothorn with Secret Sword without successfully bluffing Specs first you're a better player than I am haha.
-I never considered Heracross, it would certainly help the matchup vs stall! Furthermore it baits the hell out of Talonflame, which is great for Kabutops, though be careful of WoW 'Flame and Pinsir, both of whom can own tops on the switch. Heracross doesn't like facing offense much at all, and its frequently being forced out is a problem for teams lacking solid defensive synergy, but when healthy it handily puts Thundurus down too so that's always nice. Seems niche but I'll definitely be testing him.
-Thundy can do so much for rain it's not even funny. Unfortunately (i mean not really but you know) what it wants to do most is check/lure venusaur/ferrothorn/ttar, so you REALLY want both of Fighting/Flying coverage and LO. I suppose NP & Thunder would work great in the last slots (and make you need LO much less), since full coverage NP thundy is a pretty rare sight and can net you some sneaky KOs while your opponent scrambles for a check.
-IMO Scizor and Mawile are about tied in usefulness for Rain. SD Scizor just forces so much shit around and is one of the only usable pivots for HO. Unfortunately its countered really easily, and while it sets up on Ferrothorn and Chansey it always loses to something on a team with those two in it. Mawile has negligible defensive use in comparison (hail almighty M-Zor bulk), but all its switch-ins are begging to be crushed by Swift Swimmers, so there's nice opportunity for double switching. And as Fourth Inversion said, Iron Head lets you smack down unsuspecting Venusaur, which is great. You kind of have to choose between subpunch and SD depending on if you want to beat Ferro/Venu, but still it handles them in a more offensive way than Scizor (altho not countering Aegis/Azumarill is a letdown) so I'll give him another try on my team.
-Greninja imo is bad for rain teams. It's a weird case of a great pokemon who in this case is outclassed by the thing it normally makes obscure:
Starmie. For 99% of purposes Starmie and Greninja share a speed tier, and while Greninja can handle Aegislash better Starmie has the ability to dish out stronger (analytic!) Hydro Pumps while boasting a stronger Psychic stab to kill Venusaur and Thunder to get Azumarill out of the way for good. Normally where ninja would shine here is having HP Fire to break Ferrothorn, obviously that's not helpful in rain. Oh, and motherfucking Rapid Spin. If you wanna feel like a real techy Pokemon master you can run Natural Cure and lure the hell out of Thundurus's TW, which works 100% of the time and is awesome but obviously situational. Basically Starmie is awesum
Others were discussing briefly Venusaur and Raikou. Venusaur is good on rain balance and decent on offense, but overall I feel it's outclassed. It's an okay pivot with EQ and Sleep Powder, but being unable to effectively use HP Fire or Synthesis hurts it A LOT. It's best use is coming in on Rotom/Breloom/Azumarill, but it doesn't have SR or much to take advantage of this and as we know a burned offensive venusaur in rain is basically useless. The greatest thing about Venusaur in rain though is the effective resistance to fire lol, there's nothing better than seeing "It's super effective!" when Lave Plume nudges you for 10%.
Raikou on the other hand is very good. Just please, I've tried to make it work, but don't use the Aura Sphere/Weather Ball wallbreaker set. 9 times out of 10 standard Raikou will be better for you. It needs LO not to be pitifully weak, and the hit to speed and SpDef from the Rash nature makes it totally unable to check the things you need Raikou to check in the first place. Let's face it, GFreak teased us with what could have been an awesome rain breaker but ruined it for all of us ;.;
EDIT: Kyu-B is another mon you've probably seen on rain teams. Although its usefulness has diminished ever so slightly with the decline in Rotom-W's omnipresence, I suspect it'll be back with a vengeance now that Conkeldurr is on the way out and CroCune/Phy are making a mess out of the unprepared tier. Sub/Ice Beam/Fusion Bolt/Outrage|Earth Power|Iron Head beats up common defensive cores, especially Mega Venusaur, and weakens Ferrothorn for your sweepers nicely. It's matchup v Thundurus is double-edged for rain, since many thundurus run Focus Blast and that pretty much makes or breaks Kyu-B's value in the match, requiring unfortunate scouting, but against defense and balanced the thing still wreaks havoc. Pairs v nicely with Mawile/Zor to check faster dragons and fighting types.