I have considered selling missions, game free, and mission creators get a cut. But the mission arena never seemed to attacked any interest.
HiTech
The problem with mission arena is that you are pitching your current players who are conditioned to only go to the MA and expect to be playing other players. There are many players out there who don't want to play other people, they just want something they can play by themselves when they have a spare hour. There are a lot of people who would be less intimidated just fighting AI. That would target a completely different player base. The non-multi-player crowd.
Rather than a Mission Arena, I was thinking more of a separate install, packaged offline stand-alone single-player game. Released on Steam or purchased off your website. A couple of terrains with a sets of missions taking the player through a series of career progression missions. "Aces High: Air Combat Anthology"
Terrain 1: Battle of Britain
20 missions career progression; RAF pilot starting with Hurricane's and the being transferred to Spits. Big finally, massive dogfights over London!
Terrain 2: Stuka Pilot
20 missions career progression; Barbarossa to Kursk. Attacking tanks, airfields, shipping, refugee columns.
Terrain 3: Cadillac of the Sky
20 missions P-51 career progression ETO; Bomber escorts. Fighter sweeps. Airfield strafing.
Etc.
Sorry. Apparently I've had way too much coffee this afternoon.