Sounds kinda like Armor Digimon in the sense that different Digieggs mixed with different Rookie Digimon resulted in different Armor Digimon. 
That would be a bit much for Pokemon though. I'm not fond of a single Pokemon being able to change forms (I don't mind Pokemon available in different forms, just having 5 Rotom, 4 Deoxys, 17 Arceus, 3 Kyurem, etc. is a bit much).
That would be a bit much for Pokemon though. I'm not fond of a single Pokemon being able to change forms (I don't mind Pokemon available in different forms, just having 5 Rotom, 4 Deoxys, 17 Arceus, 3 Kyurem, etc. is a bit much).
It's not changing form, it's turning it into an entirely new species. Hidden species, basically, based on the precursor species, but of a new type or element. If you could only do it once then you could only acquire a single re-engineered specimen (which probably couldn't breed true or be traded, either) making the event a more varied and exclusive variant on the traditional fossil choice. Do you want the water/flying version of your Charizard, Deluzard? The bug/electric version of your Growlith, Barktric? Ghost Hitmonlee, Brusoul? Or the dark/dragon Gyrados, Arashidos?