This can be blamed on Bomberman Quest.
Megaman's VWS/Cross System (haven't decided how far into MM9 this takes place) finally comes back to bite him in the ass. This particular program, based on the theory of evolution, reduces him to a pathetic blob within minutes. However, there seems to be a way to "work back up" to his old body...
PATHETIC SLIME: Compresses poor Mega into this form. Unable to use his buster, he has to headbutt enemies (small ones only. If he tried this on Zero...) to defeat them. He has only a fourth of his original power as a slime. (Derived from the original Bombslime)
DEFORMED: A little better, but still vertically challenged. Megaman regains use of his buster but its power is cut in half; jumping is also hindered. (I finally found use for the MM8 super-squats)
TERRAIN: Megaman now has a more humanoid form, but the armour sucks. Well, almost there. His buster has 3/4 the power of the original. (Derived from Captain N; they actually made him blue/cyan for Season 3, but the animation was so awful by then it wasn't worth it)
REGULAR: Not much to be said here.
ENDURANCE: Imagine Mega's surprise when he finds out there's a positive side to the evolution program. As the name implies, Megaman can take more damage than any of his previous forms could, plus his buster is now stronger. The added height is slightly disorienting at first, though. (Based on Ruby-Spears Megaman. Anyone who's been following my continuation will know why this is something of a disturbing thing)
SOLDIER: Tougher armour and and a power boost--not much to say, but the best is yet to come. (Based off of Eddie LeBron's interpretation of Megs. I hope he doesn't mind)
EMPEROR: This is why the evolution program was even coded. It's the final stage, and the strongest--there's little that can stand in Mega's way after this. Imagine getting to use the Hyper Adaptor in Megaman 8. (Unsurprisingly, it's based off of MvC/2's Hyper Adaptor)
Even after seeing the power of the Emperor, Bass refuses to copy the program himself. Something about not wanting to be reduced to a pile of shit.
*I wanted the contrast between the Slime and the Emperor to be akin to looking at Koromon and then Greymon, skipping the middleman. Did it work?