The EXACT same as Single player but with humans.
Agreed! Well, I would actually make one tiny adjustment (as an option): Allow "simultaneous" turns a la Civ 4 or AoW: Shadow Magic, that way turns are only as slow as the time of the slowest person, instead of as slow as the time sum of all the players combined.
Cloning the single player experience has the added benefit of allowing the developers to focus on the single player development, instead of developing two different products. Some people might find that turn based games are bad for multiplayer, but those who buy a 4x TBS with multiplayer should already know what they are getting into. When I buy a 4x TBS that claims to have multiplayer, I tend to think the multiplayer will also be a 4x TBS. I don't usually expect wizard duels!
Any other fancy modes the developers want to add should be extras, not the main event.
Sadly enough, Brad doesn't seem to want this. (Well not short term)
His "Multiplayer" essay just drove a dagger through my poor little TBS-FANTASY-HEART. 
I found his concerns to be overblown and misguided, catering to a competitive element in a genre that isn't known for that is kind of weird. I know there are Civ competitions, but it's nothing like RTS or FPS. Those later types of game rather require competive play to be possible, as TBS games should be built for friendly gameplay. You are not going to play a 20 hour game with some random stranger "overnight".
And then he talked about Wizzard Duels *Shudders*. That really, really scared me. Like WTH?
I can't talk about this anymore it's too upsetting lol.