To me, it seems that the upper left path is really the only one, with immediate benefit, typically.
Because that's where crit and multi hit, are. And unless the card is an LR or so, unlocking the special attack path, is not worth the resources; necessarily speaking.
This is, especially prevalent if you have to awaken a card in order to use it, to open a path. Like buu piccolo.
So maybe have a buff for the cards passive or a 10% addition to the leader skill, for maxing out a cards hidden potential, or per a completed path.
EG, butterflies bluma, gets an 300, added to her rainbow ki health recovery or phy super trunks an 3‰added and 1 more ki.
It would certainly reward, the most dedicated, and could make personal favourite cards more viable, like TUR str yamacha, that I adore. That also had a pretty op passive, that would be quite potent on top tier cards, like vegito, broly or any LR.