As far as game rules:
I would say no AI players to start. 12 - 18 players is a lot to deal with already, and in a game like that starting near an AI is a HUGE advantage because the AI is generally eaten by human players ASAP in this kind of situation. Besides, the inevitable drop or two will leave an odd AI here or there, but those will be up for new players to grab since they were player started.
I would NOT be in favor of Raging Barbs, and would in fact not be against turning barbs off in a competitive multiplayer game. There's just too much chance of one guy starting off next to a huge tundra that spawns dozens of axemen for my tastes. Fighting barbs is no fun when everyone else is wheeling, dealing, and fighting humans.
Tech trading... I can go either way. Should we consider the compromise that others have developed, where by you can only trade techs that you yourself researched?
I would be in favor of playing on Monarch or Emperor. There are no AI civs to get production bonuses, but it makes the number of free happy and healthy citizens smaller which will increase the importance of trade. I find things like that make multiplayer much more interesting.
Map size:
I'd suggest either Large, or a standard map with little water. Probably lean toward Large though. Not huge, because we want to meet people as soon as we can to get into the meat of the game.
Map type:
I would personally prefer Tectonics, but there's a big chance someone would get shafted by a crappy start. Thus, I'd say either Fractal or SmartMap.
The only victory condition that does not appeal to me in this kind of game is Space Race, but I'm not too concerned about that.
