I'm thinking RC + SS just to make it interesting as possible. But ultimately RC + spread out is fine with me.
"As interesting as possible" would suggest to me "no RC + SS" - do you mean to suggest we should or should not have AI major regimes? I'd assume you mean we should.
It occurs to me that a strong point in favor of having 4+7 majors is that it would put a hard limit on the possibility of snowballing via charismatic foreign affairs directors, which was definitely something we saw happen in our last game. As soon as a minor goes to war with any major it can't become a protectorate/vassal, so having 11 majors would mean wild expansion would require actually expanding for the most part.
I'm wary of going in too heavily for seas - they're hard barriers that can only be directly bypassed via airbridges, and that's quite an undertaking. OTOH, mountains are soft barriers once you get your industry rolling - IP smashed through very deep mountain ranges in the prior game. Likewise swamps, jungles, and canyons.
I'd be happiest with xeno or terran ag being possible even if it might not be ideal conditions for one or the other. That rules out magma and ice planets for the most part. I'm sorry, but I have a sick, disgusting areal fetish and it really revs my engines to see farmland spread and supplant native forests. I'd also like fairly advanced forests just b/c it's less boring. Ideally, I'd like a pink, teal, or taupe plant color pallet, but that's really being finicky. I'm willing to hear opinions, though - if someone wants red or turquoise seas, or borderline-fluorescent-green or yellowish trees we can probably make that happen.
I'm leaning towards mid-sized megafauna - a max of 10-15m is probably the safest, and that's actually enough to create some horrific death beasts. It also makes it easier to avoid having too low gravity, which tends to lead to fewer minerals to mine. Moderate gravity and reasonably thick atmosphere will allow for flexible aircraft options as well.
Any other thoughts?