I just hope it's not WWII because people are bored of it. Now it's all about modern combat. If TWI wants to be original, I would suggest realistic futuristic apocalyptic combat. Like Mad Max but then with tanks and large scale combat.
If TWI will put enough effort and money in marketing for their next game, then a modern combat simulation would probably become very popular. Come to think of it, there hasn't been a realistic modern combat multiplayer game yet. Well, there's INS, but we need something that isn't Source.
I think many people want such a game, remember how incredibly busy the INS forums were on the day of release? Or just look at the popularity of ArmA.
However it would have to be a very special game in order to beat ArmA from its "modern combat simulation" throne. Same thing if they make "RO2"; people are getting tired from WWII by now like you said.... so it would have to be VERY good.
Futuristic realism would also be pretty cool, agreed. Then again, there have already been a few games based on that setting.
Personally, I don't really care which setting their next game will be in, but if they stick with the FPS genre I do hope that they make the level design on a 1:1 scale ala Kriegstadt or ArmA. And if it's multiplayer, teamwork options should be a big priority.