Literally all you need to do is make a shooter where you pick your frame, pick your internals, select 4 hardpoint weapon/utilities, and then fight either in a 12v12 or more over a large map with basic offense/defense objectives like holding ground. Then slap on a sidemode where you instead fight waves of enemies.

Literally all you need. That idea's free, you're welcome, shoot anyone who tells you to make an extraction shooter.