Scarf Keldeo is entirely irrelevant, as is kabutops, as are 404 HP subs, as is stoutland, as is singles experience, as is tanking a partner drill's EQ, as is 252 HP Lando-T... do you even play doubles at all?
As for my personal numbers, I'm looking heavily at the ponies, because they have extremely strong attacks that we want to switch into, and because Terrakion's sandstorm boost means we'll need some power to KO it. Not even 135 SpA Modest with Life Orb guarantees an OHKO on Terrakion with Earth Power, so I think maybe we should force ourselves to use Leaf Storm. Defensively, we might want to be able to switch into Close Combats and Hydro Pumps from the ponies—but it would take a good amount of bulk. 252 Atk Life Orb Terrakion Close Combat vs. 0 HP / 0 Def (95/90 custom) : 277-328 (83.6 - 99%) -- guaranteed 2HKO. Notch that down a bit on the Special side, since HPump is slightly weaker. We'd need bulk higher than Terrakion's own to tank its hits, with a Speed also higher than Terrakion's. And it's pretty hard to overrate Speed. We might have to make sacrifices somewhere.
Consider this: Venusaur statistically only has "Good" special sweepiness (135), while Ludicolo is even just "above average" (115). However, the two of them also have additional important boosts that our much more straightforward CAP will not; Ludicolo gets Fake Out and Rain boost to his Hydro Pumps; Venusaur has Sleep Powder. So perhaps the best comparison for us would be Excadrill, who is also very straightforward and has a sweepiness of 190 (Excellent). It's worth noting that we have much better STAB coverage than Excadrill though, and indeed, probably won't even need HP Ice—252+ SpA Life Orb (110 SpA custom) Leaf Storm vs. 0 HP / 0 SpD Landorus-T: 325-383 (101.8 - 120%) -- guaranteed OHKO. We could very feasibly run just our STAB coverage and a utility move as well.
Another thing to note is how the scenario could work out in battle. Things that are short of a KO for the CAP could very well be "chipped in" by Tyranitar who will basically just be spamming Rock Slide next to the CAP, discouraging any of its counters from actually switching in. This Rock Slide spam is a powerful dynamic which can be exploited to pseudotrap the opponent by making switching difficult—something that we can already see as the main strength in Sun teams between Zard's Heat Waves and Venusaur's Sleep Powder.
That was a bunch of completely disparate thoughts to perhaps jog discussion or at least give everyone something to think about. Between OHKOing Terrakion or Aegislash with Earth Power, switching into Terrakion, switching into Keldeo, we obviously want to be able to do all of these but are going to have to choose because being extremely fast is pretty fucking huge in itself. Right now I'm looking at PT and ST limits that will live the musketeer attacks and a SS limit that will allow a wide variety of Speeds and Special Attacks, but a pretty restrictive BSR to make sure that no one Pokemon can do it all.
