The one thing I think we may need most is dead civs resurrecting. I know it goes against a faster mod, but there should be a way to balance it all out so that this lets us make civs more aggressive and dangerous without being afraid they'll wipe everyone out (but so they still wipe some out).
Something like the following rules may work:
1. Random chance of resurrection exists only within 20 turns of elimination. It goes up quickly - turn 1, 10%; turn 2, 20%, etc, for five or six turns. Then it goes does just as quickly and remains at 5% or 10% for the remaining turns period in the short window. This percentage chance per turn is the base chance.
2. If there are any Barbarian cities in the civ's core land in a given turn, chances are doubled for that turn. However, this doubling can never happen twice in a row - if it happened last turn, this turn is normal, but next turn can have it again.
3. If a foreign capital or Forbidden Palace is in the civ's core land, its chances of resurrecting is halved. This halving is a yes/no question on a turn-by-turn basis. For whatever duration a capital or "second center" is in the core area, chances are half as big.
4. When a civ dies, nationality will have to remain for resurrection to work (it can assimilate super-quick once the window closes I guess). If any city in the core area has 100% the fallen civ's nationality, chances are double. If no city in the core area has a majority of the fallen civ's nationality, chances are halved.
5. If any units or cities have flipped to a new or resurrected civ in the whole last round (anywhere in the world), chances are double for this turn. This doubling stacks (so a conquering empire can conquer quickly, then get hit by one resurrection, then by three more, and then collapse to barbarism to later possibly return as a minor civ.)
If a random resurrection happens, every city has a chance equal to the fallen civ's nationality in it to flip to the resurrected civ. Units in the core area flip, just once, and only a small chance for each unit. No city has to flip for the resurrection to occur, it can be just a few units really.
Something along these lines will be realistic enough and most importantly create the dynamics we want, with civs dying and reappearing and changing and not being such big fat monoliths. If a civ can resurrect, we can feel free to overpower Greece, Rome and Mongolia insanely, since resurrections will assure us that at least some of their damage is later repaired. Empires will explode, growing huge in just a few turns, and then suddenly grow tiny as new and old civs take chunks out of them and likely cause them to collapse. Then later they can respawn, more moderately-sized. We can even increase the chance of collapsing if cities flip to a resurrection.
I'm toast, it's 4:00 AM and I slept badly last night (not to mention this was a long and tiring day.) I can't even proofread my post, which I customarily do. I haven't done this in a while, but if my post stinks, itsh cuj ahm reel tard!