Looks awesome.
You shuold try to make Goku's move based on Smash Flash 2 Goku.
For the Final Smash, Goku can be the only character that can get another Smash Ball(s) while the Final Smash is being used and active.
For Example:
First Smash Ball: Super Saiyan 1
Second Smash Ball (while in SSJ1): Super Saiyan 2
Third Smash Ball (while in SSJ2): Super Saiyan 3
Fourth Smash Ball (while in SSJ3): Super Saiyan 4
no
SSJ1,3,4 will be 100% same dmg/speed and anything else
it will be just like as vegeta that got SSJ1,4 in 1 SSJ mode
so you can choose what form you wanted for SSJ mode
and SSJ mode will be actived by got some dmg
not by FS since thats dragonfist or spirit bomb
if i do something like that idea
so it will end up like SSJ4 need to have like 50% dmg on the weakest attack if SSJ4 needs to be stronger then SSJ1,3