In my game there are two teams per PvP match, and those two teams are further divided into air/land fields. Each team has 12 players on the ground and 6 patrolling the sky. The goal of the game is for the air/land units to work together to win the match. Players are matchmade into games randomly, without taking into account skill/field preference/etc (the short explanation is that we don't have control over this).
If I allow complete freedom in choosing fields, I might end up with no players in the sky or vice versa on one or both teams. If I don't give players a choice, they won't be happy when they aren't assigned to the field of their preference. Overall I imagine the fields would balance out on their own if they're equally enjoyable, but there's little room for error there -- if either field gets a bad first impression, it'll be difficult to recover from. I want to ensure there are some constraints in place to prevent death by conjecture.
I thought of adding 1 or 2 landing pads at either team's base, and that way players who wanted to play the land field could land, and players who wanted to play the air field could take a parked aircraft on the landing pad. This could breed tension between players where more than 2 players are competing for landing pads though.
What are some approaches I can take to resolve this, which avoid forcing player behavior as much as possible, but still ensure gameplay is predictable and reliable? Specifically I'm looking for what key issues an answer takes into account and how it resolves them. If you're drawing experience from something that's already been put into practice and already has tangible results, those results would be a wonderful addition to your answer.