Battlefield games, and the entire FPS genre have changed a lot from 1942 and BF2. The Bad Company series polished infantry combat, vehicles, destruction, and many other Battlefield aspects. Many would say Bad Company 2 is the best Battlefield game yet. Sure BC2 is coming out on PCs, but lets face it, BC2 is not what we would want from Battlefield 3. But what do we want from Battlefield 3?
 
DICe has the huge job of making BF3, by making it bigger, while incorporating some of the stuff from BC2. BF3 will have to have everything BF2 had, but more, bigger and better looking.
 
Having a 64 player cap is so 2005. Games have advanced a lot from there, so having a cap at over 100 seems like it wouldn't be too much to assume, despite DICe seemingly scaling back the scale of the battlefield games in favor of close-quarters combat.
 
Maps: The maps are going to have to be even bigger than BF2 to support the increased playercount, but at the same time look good and be interesting. They should also try having more CQ stuff.
 
Destruction: If it is frostbite, it is going to have it. But will it be better than the destruction in BC2? Maybe... maybe not. BC2 is only 32 players for the PC, DICe will have to take significant steps from there to BF3 in terms of destruction, and with larger maps, the potential for over-stressed servers is a major problem.
 
Vehicles: Obviously the return of jets is a must. And with an increased playercount there can be even more vehicles, such as UAV predator drones, bombers, helicopters, and all of that good stuff.
 
Game-modes: Lets be honest, BF3 needs to have more than conquest. Would they bring over Rush from Bad Company? Will they have some of the other modes from Bad Company? I think we might see some variations of Rush, but DICe is going to have to think about what gamemodes would suit such a game.
 
Structure, map design and vehicles: If BF3 were to have a playercount of 100+, UAVs, jeeps, tanks, jets, cobras, blackhawks, they have to think of a way to balance everything, balance the maps for the vehicles, optimize vehicle spawn points, vehicle damage, vehicle health, rpg damage. 
 
Getting people to be team-players and work as a squad in a game with such a large playercount, maps, and vehicles is going to be very challenging. DICe should look to MAG, but I encourage them to improve on it immensely. 
 
DICe has a pretty big project on their hands, and I hope to the gaming gods that they can succeed.