It is somewhat embarassing, but I tend to like battlefield beat 'em ups. So if I could make any game, I'd start by taking the formula from Kingdom Under Fire: The Crusaders, and adapting it to the 360/PS3 generation.
~For a quick run down of KUF:TC~
Essentially you're an officer of a military unit in a medieval fantasy setting. Your unit is just one of many on the battlefield, both on land and in the sky. You have some sub officers too, that do special attacks as you build up command points from thrashing enemies or using tactics(e.g. having cavalry repeatedly charge through enemy lines with an additional bonus for attacking from the side or rear of their unit, having archers attack from high ground on the battlefield, facing an enemy with the sun at your back to lower enemy accuracy and defenses, etc). Not only do you control your character, ala Koei's Dynasty Warriors series, but you can issue orders(and queue them up SoaSE-style) to other units under your control on the battlefield. Between missions you get more gear for yourself and your men, and can change their class with the right leadership points. Later on you fight massive living siege engines and aerial monsters, or even blimp-style dwarven bombers and battleships.
But instead of just having you assault empty fields and whatnot, I'd adapt the game and add a few touches from game's like Koei's Bladestorm or even Age of Empires 2. Specifically, the ability to assault large castles or even towns. Enemy fortifications would make sense too, such as having archers and ballistae on the walls to assault your units from afar. Others would be circled by moats, with a portcullis at the entrances that would need to be knocked down. Alternatively, you'd be given the ability to separate from your unit(a la the bodyguard orders from games like Dynasty Warriors) and infiltrate those places. Perhaps an aerial unit will pick you up and drop you on the walls, where you'd descend into the place and take on the guards from within. But that also means that you'd be able to ride aerial units, making the game somewhat like Lair in that regard. Or, for those that really want to focus on the command aspect, you'd be able to ride inside one of the aerial bombing or battleship blimps. When in such a unit, your subofficers would provide bonuses for the ship, and be able to use their spells from the air. The idea is to blend aerial and ground combat in a way that the player can participate in either according to their whims.
Since a number of games thrive on multiplayer these days, there would be a Kingdom Versus mode, where humans could control opposing armies and send them against one another. Perhaps you'd also get a number of Defense Points to add traps and extra fortifications to your main base. Obviously good management of unit deployment formations and application of archers, mortars, and cavalry would be key to victory. But for the co-op minded folks, there'd be a Fortification Assault mode, where multiple players take on the role of officers in a larger army, all with sub-units of their own, and attempt to take on a massive AI-controlled fortress. On the flipside, there would be a Fortification Defense mode where players attempt to hold off a massive hoard of monsters.
Overall, think RTS meets Beat 'em Up.