see how you have a pool of abilities when cyclcing through elements. Player starts with only pew pew, present an X number of abilities at once(3 Usually) every Y minutes, tied to an economy system (usually enemy moneys drop), and let the player decide which one he would like to pick. You could this with passive bonuses as well. Check SNKRX to see how that shop is done.
This is exactly what I was planning on doing lol. It's probably going to be tied to general xp and be a level up thing though.
Thanks for making me feel better about the idea! There are a few ways of doing it (ie relic pickups like RoR2, discrete choice Stat upgrades like Diablo, skill tree etc) but I think the pool is best for this game since it'll be multiplayer and it'll mitigate meta builds