Map needs to be Pangaea. Isolation is a real problem in multiplayer civ - if someone has no neighbors they can very easily run away with the game while others are fighting, and Pangaea is the only (non-DLC) map type that mostly stops that from being the case.
Difficulty should be Prince, King or Emperor. Since there's no AIs it doesn't make all that much difference, but I'd suggest King, since I think that's the level a lot of webDippers are at. I think players should be required to play on until either:
- they are completely eliminated,
- another player has won some victory type, or
- 2 players both agree to concede to the third.
This stops the problem of AI neighbours being too easy to conquer.
No Raging Barbs (the main adversary in the game should be other players, not the environment - normal barbs are enough), no policy saving (this makes every tree except Tradition/Liberty and Rationalism practically worthless since the best strategy is almost always just to complete your starter tree and then save for Rationalism).
Pace needs to be quick or games will never finish (as Wjessop said - quick games tend to take 8-9 hours to finish in general, standard games would take even longer).
No preference on sea level, I'm happy with strategic balance, and I'm happy with Tiny map size.