Soul stealing, Soul mixing, Soul overload (because of quantity of attacks used), a magic metter ( helps about the soul overloading), Soul draining (when you use all your magic u will start using your soul vitality eventualy breaking it if used too much, Improved effects, Make a tier system for souls, monster souls, and mixed souls, being able to use magic much more effectively and powerful ones. sorry for my bad grammar too lazy to fix this stuff

add tiers to weapons too, Being able to surpass the defense of certain tiers: for example, a tier 5 sword could affect a tier 5 being (or soul)

There could also be a LV system, But i dont know how to make it so goodluck!

so sorry