Have you also considered making the super consist of (1) several stages, (2) adding a new sequence of inputs for it that cost more meter or (3) any combination of both? If so, what are the disadvantages or advantages of both in this context?
I personally think that 236PP > [P] > [P] for a triple-stage super would be interesting if possible; You could use the startup time of the super to check whether the player is holding P, enhance it, then have another startup period that checks again for the lv5 but I assume at that point you won't be able to discount the super if it were Sequel'd into, ditto for (1).
If not that, something similar to Xiangfei's BC > 4,3C > 4C would be interesting, but might have the aforementioned cons?
[PP] or [KK] would also be a set. 6PP, 4PP also would be neat. Another idea would also be to make it depend on how LONG you've Held PP; I make wild assumptions about the game here of course, but perhaps store the time you've been holding PP after a certain period in a value, if the value is over treshold A, give lv3, treshold b, give lv5? Of course there begs the question of displaying how much charge you've gained and representing the effects.
IIRC Examples for the above are like
(1) Xiangfei's SDM
(2) Bullet's 360 > 720 > 1080
(3) I'll get back to you on that.