well, I mean are you really saying you'd want things to be placed on the map in real time? L4D has versus, but its hardly giving the player control of the master AI, in fact, quite the opposite, its giving the master AI more control of the player (forcing them to change characters and respawn as tank and so on)
I'm not saying its a bad idea, I'm just saying it doesn't really sound like... this game. I mean, if something is going to be added to the map, I want it to be an event or something like that. I don't think the story/events should be designed for human control, it just seems like the wrong direction for me. I especially think it would downplay some of what the players have to deal with as being channelers. By putting the world in the hands of another player, it changes the scale that the channelers would have be working to achieve. I can't speak much for what exactly Stardock wants players to 'feel' when they are put behind the reins of a channeler, but if they want the world of elemental to feel alive and mysterious they can't allow it to be just another player.
I'd be down for giving a player control over all the neutral factions, random mobs, and such as that, but I'd imagine it to be really boring for them since they'd loose half the game.
GM players create a very different atmosphere from AI or mechanical opposition. Look at board games like Decent (GM player) vs. Arkham horror (mechanics opposing). The way the end goal is viewed by the players is very very different, and not just because you loose arkham horror half the time, its because no matter how tough the enemy is, when it becomes another player it takes away its fangs and leaves it entirely to the players to put them back. (and lets not even get into superiority complexes created by being allowed to play the GM in such games. I've stopped playing certain games with too many friends because of this effect)
There was a really good interview with some blizzard devs about why the introduced the burning legion in warcraft 3 for the reason (its where I get the metaphor 'fangs' to represent player respect, since they talked about how they didn't want the burning legion defanged by letting them be playable.), sadly I can't find it among the crap that is most WoW story content and interviews.
(sorry to any WoW fans, just my opinion, take it as nothing more. I will stand by the fact that WoW has destroyed the eredar and the burning legion's image created by warcraft 3, but talk with me about it elsewhere. I'm just using the example)