it could be as simple as a more advanced morphs.json config, like being able to tell the cfg exactly what "fireball" to throw
an example of what would need this tweak is the Blizz, which is a blaze variant added by thermal expansion. it shoots fire, but it uses a different sprite, so just using the default fireball wouldn't quite fit

This should be done through mod add-ons. Metamorph exposes an API which should allow adding custom abilities, attacks and etc. I'm not interested in developing such feature as it's quite too vague described and doesn't seem to have any whatsoever general use.