I'll throw my opinion as someone who has been a part of every previous SPL and has seen every format in practice.
10 slots is far better than 12 slots. Leaving aside the "you get more shitters in the tournament" argument which might or might not be true, it also means, as someone already mentioned, that the contributions from one player to the final scoreline is diminished by 20%, thats huge. This would theoretically be "fine" (still isn't) if the funds from credits were allocated to players with this in mind, aka top players costing less because of a lesser contribution to the week scoreline. Except this will never happen because 12 slots also means more credits, which inflates prices, which makes top players more expensive. So you end up with an awkward situation in which the top players are really worth "less", but they cost more. Undesirable. 10 slots should be the only way going forward. 11 slots shouldn't even be discussed as it's terrible.
So what do you kick out? blunder really has the best post in the thread and it should 100% be 3 SV slots at the minimum for this tournament. 3 would be the best, I don't particularly like the idea of making the tournament basically 50% current gen OU (counting tiebreaks here) since that's not what SPL is supposed to be (hell, even in the wild west days of SPL 1 we didnt have 4 CG OU, imagine). No johns is stupid, no point discussing that. Blitz is more interesting and I've always been a fan of it because I feel calculating and making a gameplan fast is an actual skill, but I can see why people wouldn't want it. In the end, I feel like I prefer removing a tier (and saying this as a boomer that has always defended old gens in every tournament) than adding a 4th SV slot, and if you add it I think I would prefer Blitz over a normal one (if youre already going bonkers by adding a 4th CG OU, going against SPL history then might as well give it some sauce). I think the removal of SS is a crazy idea but honestly, it might just be the way to go. It's weird, yeah, but if you think about it a bit more it kinda makes some sense. The only gripe I have is removing the most played tier in the past 3 years on Smogon's best tournament but eh, might be worth it. Coconut's point shouldn't be considered as you just return to 2 SV the next year and everything's perfect.
Bo3 and Bo5 works in random forum PLs because noone really gives too much of a fuck and they just reuse teams. In SPL people actually care and can spend ungodly amounts of time building and testing, so I don't think it would work too well. People can just reuse teams here though so maybe if you're fine with seeing the same shit all the time in the tournament then its okay. Still prefer the other options if you're going with 4 SV and like I said already, 12 slots is so inferior to 10 that meh.
Doubles, VGC, Uber, and whatever other lower tier you might think of shouldn't be considered for obvious reasons.
So in my head:
3 SV + Removal of SS > 3 SV + Removal of RBY >>>> 4 SV with Blitz as the fourth one > 4 SV > 4 SV with BoX >>> 11 slots >>> Anything including a lower tier.