So Elemental is destined to have multiplayer, which means it will run in to the same problem that every turn based game has when more than one human player is involved: one player will take longer than other players to complete their turn, leaving the other players waiting. Even if there is a time limit, slower players are likely going to run the clock down to the last second.
I personally think that if you do have simultaneous turns, you need to provide as much incentive as possible for players to play efficiently and not soak up time (or disincentives for taking too long.) On the same token, you want to give people who have already ended their turns something to actually do with their time while they wait rather than just quo up unit movement or construction. So here is my idea...
Players that finish before the last player hits "end turn" take part in their own mini-game which would represent the sovereign having extra free time on his hands. Success in this mini-game could improve spell research (sovereign studying notes and texts), augmenting mana supply (attuning ley lines and magical flow) or even improving hero recrutiing (sovereign personally courting heroes at pubs and inns.) Naturally, players who take too long don't take part in the minigames very often (represented by the sovereign overmanaging their empires and not having free time to do extra research, etc.)
This system would reward snappy players and encourage slow players not to re-check every single city before they hit "end turn." And of course, it keeps everyone busy so that no one is waiting around for the (s)lowest common denominator.
Naturally, the mini-games couldn't be too easy, otherwise they would be "gamed." They can't be too complicated either. So ideally, each mini-game session would terminate each time every player has hit "done" with their turn, and new minigames would start. So someone playing the minigame would have to make an estimate as to how much time they have left to work with. If they decide to go for broke and everyone end's their turn pre-maturaly, they could lose all that they gained in that specific mini-game session.
Ideally, the mini-games can be influenced, too, by the "landscape" of the game up to that point. For instance, if you are pursuing spell research during your free time, the mini-game's own "landscape" would be based on what you have researched so far. It could be a "stealing" mini-game where you try to reverse engineer other player's spells or it could be an "extrapolation" mini-game where you strive for a creative breakthrough based on your own spells.
In the end, it would be a fun, even suspenseful, way to keep people busy in multiplayer while other players are taking their turns. At worst, the players could always just turn the feature off if they don't like it.
The mini-game's parameters would be different depending on the the activity and the player could choose which activity to enhance. Players who finish sooner get to play the minigame longer and enhance that goodie more.
Naturally, it wouldn't be a huge bonus you'd get, but it'd still be quite an incentive to play efficiently.