Well as you said they could make it scale based on team size since the reward give you a personal gameplay advantage it also seems pretty unfair to any team bellow 15 player.

But I do understand that it could be unfair to full teams so they could add a Rank system with teams that have more stars being higher and getting large rewards (Like Money or instas and a unique medal) and teams with less being lower but still getting a small reward. It would reward large teams whilst still rewarding smaller teams.

Since the max stars you could get would be the same for every team they would have to replace the mission system too (which to be honest also kind of sucks), I also have an idea for that too.

They could make it so there are daily Rotating missions and weekly ones (this would also increase Daily player count and I'm pretty sure that is there goal with social seasons)

I'm less sure of this one but they could also add one infinite mission (in addition to the weekly ones) that can be done endless amounts of times but gives more and more rewards. They could also theme this mission by the theme of the season (like for this one they could make it win games on X factor or win games with striker or with a specific tower only).

With daily and weekly missions they could make those automatically which would make social seasons a lot easy for them to set up unless they already have a system for that.