I recently used the cross breeding in the game to get new varieties of angel fish (see the tank in the workshop). I was able to get reasonable facsimiles of black marbled and blushing angels. It would have been even easier if I could have specified I wanted certain aspects to come strictly from one parent. In particular, I wanted the body shape and fin shape of the angel fish. The coloring I wanted as a mix between the angel and the donor species.

Also, as far as I can tell, you can't cross breed a fish that is a result of a cross breed. That would also be very useful.

I know the list of features people want is probably already unmanageable, but I think those upgrades would really make an already good feature shine.