An RTS/RPG/FPS might be cool. You could start out as a soldier or low ranking officer, then rise through the ranks with an experience base level up system. You could upgrade your tactical effectiveness with different unlockable bonuses that can be used during battles, and specialize in different areas of combat (special ops akin to Republic commando or rogue squadron, fleet/ground commander eaw style etc.). Your squad/legion/fleet could have customized design options. Additionally, for another rpg twist, the campaign mode's story could be affected by certain decisions you make (think star fox 64, but more complex and up to date). You could also defect, perhaps sell information (with the risk of being exposed). As say, a fleet admiral, you could promote or demote different subordinates based on their tactical prowess (think kotor party management mixed with the tech level upgrade system in Age of empires III/ Age of Mythology).

Each mode of combat (e.g.: BF-like skirmish mode, Republic Commando-like special ops squad mode, tactical mode, etc.) could also be played sandbox style, either through a main menu or an ingame option; for multiplayer or single player.

Thoughts?

Seems like the ultimate game... given infinite amount of development time, it is certainly possible. There have been many hybrid game approaches, sadly they are good at none of their genres and thus do not make great games.