I always wanted a game in the style of Warhawk mixed with MAG's 256-players-online, but in a Medieval/Fantasy setting.
There wouldn't be classes. Your character is capable of any actual ability, but you have to get pick ups along the way (ie. books for spells, swords, knives, axes, maces for melee, long dark cloaks (or something similar) for stealth, and bows or crossbows for long-range combat) in order to fight.
Characters would be customizable. You'd be given the option to create a character for each faction, so that in team-based game modes, you wouldn't all look exactly the same. The differences in each character wouldn't be so incredible that you could have some huge, heavily armored knight fighting alongside a bum. Body type, facial features, accessories (belts, pauldrons, helmets, tunic, tabard) would vary, but chest-plates and leggings would be the same (except for maybe details like dents, scratches and stuff like that).
Combat would be balanced. Melee weapons would have high amounts of health and do the least damage, but when up close, it would be a barrage of hits. Ranged would do medium damage, but have very low health compared to everyone else. Magic would be the slowest, have medium health, and least accurate method of fighting, but when the spell hits, it would do a large amount of damage.
"Vehicles" would involve horseback riding, wagons/chariots, small to large boats for naval combat, and for dragons for flight (kind of like Lair but not stuffty).
Game modes would involve your standard TDM, CTF, KOTH, CTB, Dog Fights, and FFADM including modes with objectives. For instance, your team could be trying to defend certain points on the map while the opposing team's goal is to strategically take each one, and weaken the main point's defenses in order to capture it. Game modes where only one type of weapon is available. And so on.
Maps would vary from towns, cities, forts, forests, countrysides, bridges that connect very tall cliffs, mountains, beaches, etc. All maps would be huge in order to hold such a large amount of people.