Cool idea.
I personally think the problem with nidus is that gaining mutation stacks makes you deal more ability damage.
In my opinion we should be able to choose what mutation stacks do for us similar to how chromas elemental ward works (energy color)
For example gaining more survivability per stack, more utility, more mobility, and more damage (the current default)
Because right now I don't care about his ability damage. I only care about weapon damage. So it would be nice if gaining mutation stacks increased his tank stats (or something else) instead of ability damage