As far as I know the only mutate spells we can learn are on the mutate master in Celestia. Well... I outgrew those. It is a really neat feature, I like having alternate color versions of spells. Before I had deer knight, Death minotaur was extremely useful! It was also fun to use another school's spells with my own school's damage type and take advantage of my own school specific gear bonuses.
A new round of mutates for higher level spells would be nice. Though I would like to avoid the, hit a monster and leave a 20% trap behind, mechanic. A storm or myth version of call of Khrulhu for example. You don't need to follow the allied schools mechanic as you did in the first batch of mutates. As in my example Khrulhu is a sea monstrosity but also a sort of modern day myth. That is why I suggested storm or myth as a mutate. I suppose you could also just mutate some lower level spells with useful effects, but keep the level requirement to train the mutate a high level.