I don't think there's any practical way of implementing such a feature in the game

Maybe it could be cosmetic, with no skill changes or power change. But things like gotenks's hair. Just mix up a mesh of the two characters design. this would make it fair, and more accessible to everyone. itd be like a rare mount drop in WoW. doesnt affect how you play, but its cool to have :)

well with ideology they could make a class called Metamorans which have a different varity of powers and char combinination

Yeah that's an idea, but I guess for their transformation it could just be called Fusion dance.. and when used another metamoran could just pop down out of nowhere, do the dance, and boom you get your transformation for the class. In terms of character fusions, the could implement things in a similar way but it's still a long shot off, if even possible.

A transformation to make a Hybrid build that won't be horrible yet a well rounded character in both class choices.