Louis XXIV
Le Roi Soleil
That's how my friends and I played Civ4, but it's certainly not an optimal setup. When you're 350 turns into a game and one move out of turn can win a war, some of my "friends" are jerks about it.
The absolute best option would be to have simultaneous turns during peacetime and forced sequential turns once civs declare war. If A is at war with B and C is at war with D, A+C could take turns together and B+D could take turns together. It wouldn't be that complicated to implement.
That would be a cool idea. Completely random observation, but it reminds me of Perfect Dark, when they had a slowdown mode that turned off when nobody was near you.
Of course, the biggest issue right now is lag and slowdown. Anything about simultaneous turns are secondary. I did have a war where the guy attacking me refused to cooperate for taking turns, so it made it play more like an RTS, but I adjusted. Should be fixed, but lag is first.