One thing I wish PGR had that I enjoy about Genshin is the ability to co-op with people without spending serum.
So let's say you have a really strong character, and you want to just help people fight bosses and get stronger, at no personal gain to you. Maybe you're a light spender and you prefer to use 6 star weapons so you don't care for the 5 star grind.
It would be nice to just oneshot the boss for people without you having to spend 20 serum just to participate. Make it so that you have to spend the 20 serum to get the rewards, not to just participate.