Excellent design, but personally I dont think that fusion could happen. The amount of power would be simply too much. The Old Kai warned Vegito that going Super Saiyan may shorten his lifespan. Ok lets do some mind f*ck math. Potara fusion is multiplaying the base form power levels of the fighters. Goku's last recorded base PL was 3 million and that was right before fighting Frieza. Now based off my calculations with The Devils Corpse, we placed Goku's base PL at the time of Vegito at 10 Million and Vegeta at roughly 8-9.5 Million. Now, multiply 10 million by say 9.5 million and thats Vegito's BASE PL. Now SSJ1 is a X50 multiplier, 2 is X100, and 3 is X400. Now no PL multiplier was listed for SSJ4 but me and Devils Corpse pegged it a X900 and Goku base PL at GT at 15 Million and Vegeta at 14 Million. Now, times their GT base PL's and multiply that by 900. And finally, multiply the PL of SSJ4 Gogeta by Vegito's SSJ3 PL and theres SSJ4 Gogeto's PL.
well when it comes to evolution of course. Imagine the Saiyans a million years later after Z. Goku SSJ3 could be felt across dimensions. He also unlocked SSJ4, the next evolution of the Saiyan race in his lifetimes.
Well the bodies have to be used to such large energy. Old Kai also warned that if Vegito went Super Saiyan, the energy would be so large it would begin to feed on his life force. For example if Kid Goku went straight to SSJ3 it would likely kill him. Thats why the forms are achieved through ludicrous amounts of training. It allows the body to become accustomed and used to the massive energy.