I like how WWII Online did it. I'm not exactly sure how it worked, that's because I haven't played the game enough, but I believe there were these HQ guys looking over the whole campaign (campaigns are basically the war in Europe, that restart whenever one faction takes over, which can take weeks/months). HQ suggest what towns to attack, who needs support, etc. Then you have 3 different groups on each faction. Army, Airforce, Navy. After choosing one of those you choose an "attack group." (forget what they're actually called) These attack groups have orders/goals to take over a town, defend one, or attempt to take out an attacking camp of the other faction.
In the army, you can be an Airborne trooper, a normal infantry, or a vehicle driver. Trucks are used to set up spawn points by the battlefield, so infantry can spawn closer than the attack camp. There's a limit to how close you can get, though. You can also drive armored vehicles, and use AA guns. I remember a time in a huge city battle, 5 tanks in a row came in once we cleared an area, and told us to hop on so we could get to another part of the battle. Quickly after getting off, the tank I was sitting on blew up, so it felt like a movie.

Airforce spawn at an airbase and have orders to either support the infantry in a battle, whether it be through destroying attacking planes, or attempting to destroy the vehicles and infantry on the ground. Some airforce attack groups also might have bombing orders, to destroy attacking/defending ships, or to bomb the hell out of the infantry.

Navy have boats, of course. I haven't played as them, but it's basically the same thing as the other groups. Support and attack, except in water.
It may not have a complete RTS element, and racing fans aren't going to get some Lamborghini to run over infantry with, but I think it combines many genres extremely well, and it being an MMO, some people messing around isn't a problem. Note, I haven't played the game in a while, so some of this information might not be completely true.