Hm…
Can I double down here?
Unstable Brew/Serum: Always gives Genetically Unstable/Chaos respectively, and gives a long lasting buff, that future brews extend. When the brew expires, it removes the trait (unless it’s a core character trait already, in which case it reverts it).
Make it hard to make, since we don’t want end game mutation sprees being too easy, and have a small chance of showing up as a random lab drop.
That should provide your desired goal of a mutation over time mechanic.
The second thought, which your suggestion made me think of, is slightly different, and shares a lot in common with ADOM’s corruption mechanic.
1: Player has a Mutation stat. Various things increase this.
2: They develop mutations at set intervals with scaling costs. So 10 mutation, 21, 33 and so on.
3: Mutations are randomly checked or not checked every hour or so - having 100 mutation means you will mutate four or five times before you “catch up” to your mutation score, but doing so may happen over the course of days or weeks.
3b: Things like Mutagen and Purifier change your stat, then force one instant check, potentially changing you instantly, potentially causing change over time.
3c: Checks either add, or remove, a randomly selected mutation to you.
4: Specific types of serum add a status effect to your character, making it more likely that any mutation will be in this category - This effect overlaps, so taking Elf + Alpha means that the next time you mutate is most likely to give you a trait that both share, second most likely to be traits that one possesses, and finally any other trait.
The player will gain mutation with mutagen et cetera, but will need more and more mutagen to “progress”, and more and purifier to revert.
Part two:
After you reach a certain number of mutations, you cross the threshold.
1: Threshold is based on your currently possessed mutations (since you can only have, say, 20 in the more restrictive system this is easier to sort).
2: From here, Threshold gives explicit changes to existing mutations, rather than adding new mutations. You’re not bouncing around haphazardly anymore, the blob has settled down and given you a new existence (much like brutes do not spontaneously become spitters, they progress on to hulks).
3: Threshold is irreversible without a specific high end treatment, and at this point is now self-perpetuating. You might slow the process by taking purifier (for example, if you currently like where your character’s mutations are at, and going further probably involves more body hair than you’d like), but as your mutation stat now increases on its own you will eventually continue to mutate.
So your character is mutating away from humanity, towards something new. They pick up 8 plant mutations, 5 beast mutations, 7 other mutations from various sources.
They hit maximum inhumanity and change. Primary Plant + secondary Beast = Druid threshold, player’s mutations emphasise plant-ness and beast-ness. Mutations like a vine tail, vines from the body, fangs and so forth begin to advance and improve to their most advanced state.