uh, the spirit bomb power will grow exponentially with your own power, and the charge time is more balanced. Also, a fully charged spirit bomb cant be blocked by a normal attack (maybe a full power ff).
Yes, it could be programmed to make this mode, but it would take a LOT of coding. Even a simple math c++ code takes a half-hour to make and work perfectly, with proper loops and everything. Now take into account file I/O, load times, more files becuase of a special mode, and so on. It just isnt worth it.
Brolli is considered "legendary super saiyan." However, from all the official accounts, Brolli's transformation is just USSj. He powers up from his extreme anger, and goes to ussj. In the cell saga, vegeta powers up to this level and becomes much stronger and faster than the original transformation, his power grows exponentially compared to his original form. This is all Brolli does. Not once in the movie do they refer to his transformation or does the makers of dbz call his transformtion.
simply put, HE IS THE LEGENDARY SUPER-SAIYAN, ITS NOT A TRANSFORMATION, OR EVERYONE WOULD BE ABLE TO "DO IT".