Dragonball: Celestial

In the anime, when Goku and Gohan enter the Hyberbolic Time Chamber, then asscend Super Saiyan into more of a "Full Power" state, which was basically the base Super Saiyan form, slightly stronger without the stress or draining qualaties.

The way I would think of incorparating this into the game would be, staying in Super Saiyan form for an extended period of time, (every second in the Super Saiyan state counts toward this timer), and once the time requirment is fuffiled, Super Saiyan form, is replaced with Super Saiyan Full Power which would have a slightly better boost, and can still acess all of the forms past Super Saiyan (Asscended, 2, 3, 4).

This form could be for both Saiyans and Half Saiyans because Gohan was able to use it as well, and it is assumed that the characters are in this state for basically the whole series after the Cell saga.

Thank you for taking the time to read my sudjestion