what i would like to see with mutigens and keep the game balanced. make it so you have to add the lowest type mutigen and upgrade the same to the greater. This would give it a feeling that its evolving. would also make you think about what to put in.
in short mutigen type perminent but upgradeable.
no idea how hard this would be. i think it would be cool though.
I think higher than "lesser" mutagens should also only be created via alchemy with certain lesser to normal to higher ratios, not via enemy drops or quest rewards. That plus this upgrade idea, would consequently make Mutagens a non-exploit game mechanic where players *can* put effort in specializing and if they do, get a reward out of it.
The other thing is, i don't get why the skills have slots anyway, the mutagens should be something of a separate character value - say, 5 slots that unlock over the levels where you can low level mutations and "create" higher level ones that then upgrade your mutation values.
Restriction of 5 - where it linearly unlocks at levels 5 - 10 - 15 - 20 and 25 would also make sure we can finely limit the amount of stat effects (so that 100% critical hits becomes impossible)