Expanding on MMO ...
You guys know that game World of Tanks ?
Thats what it could be like if it was an MMO, I'd really enjoy that type of advancement system based upon salvage and stuff, it would only have to be pretty slow to keep users interested.
It would really be interesting to have a game more about single mechs that you really customize and build up to call your own and build a bond with (just like in "reality") rather than switching the thing every 5 minutes or so. This could again be combined arms, form BA to Mech and Aero.
It would then be kickass to have arranged fights over planets with units of around a battalion/bi/trinary in size with ya mates, making it so that groups can come together and gather bigger fighting forces, probably led by a commander of some sort (probably player of highest rank/skill).
Rewards could be EXP (should not play such a big role, but could be seen as pilot skill and upgrade your aiming steadiness and shit), salvage in forms of chassis and weapons and also special showoff stuff like banners and paint schemes ...
It could really imagine extrem customizability as something great, I'd love to be able to paint my own mech in a certain way and have it get "elite" (more durable, better heat dissipation and stuff overt time) with all wierd banners and flipoff-writings on it (of course, depending if clan, housewarrior or merc.
You could (in theory) even make it free to play to get a huge base of players because most people would probably not mind real world advertisement in the salvage/customizing menus.
This would atleast pay the server cost and replace the monthly fee that would annoy most people away from the game.
Combine this with great, simulator style gameplay (like I said in my last post) and it would be the shit, and something new and fresh that keeps people going.
Don't get me wrong, a L1 player should still be able to beat anything if he's good, but the ranking system would kinda give it a new kick when used tastefully.