There is no reward for playing well.

[Claustrum]

How do you define "playing well" in measurable numbers?

You want me to design the system? I can give a rough draft, but it'll still be a bit unfair. An oversimplified version would be whoever does the most damage has a chance to not lose a star, though that would favor DPS. A more complex version would check to see if you're using your abilities effectively. Or something that would make me happy is if someone spends more than half of the match in the base, they lose 5 stars, and the other 4 players on the team lose none.