The only problem is that mutations are overpowered unless limited. That's why only Rahn and the Genetic Mutator were able to mutate things in MO 2.0.
Well, possibly a hero for a different subfaction or some tech building tech or mission related.
If a mission, it could be about Allies or Soviets discovering an Epsilon genetic research center in Africa where they have been experimenting with the wildlife there and created some deadly results. Their mission is to eradicate the entire compound and setup a quarantine of some kind. Of course if this turns out like a mess of stupid shit with Zerg manifestations fighting soviet tanks it would just ruin the entire atmosphere. If done right, one might have something interesting coming up here.
But if it lags the game like Mooman's cow's in YR: Squared then it's not even worth it.