So take some character who's a Mutant, explain what you think would be a good powerup and why.
Can't go over all the times it's been done in comics, 'cause there's way too many examples from the comics to list all of the times it's already been done.
But two highlights are Angel becoming Archangel due to being a Horseman, or Beast becoming furry and stronger with a secondary mutation... Then there's how the main difference between Magneta and Magneto is simply scale of power. Magneta just can't do as much with her powers.
Well an example from me... I recently pondered how similar Arclight's powers are to those of Whitebeard in One Piece... If Arclight could push her powers high enough to affect a small country all at once? yeah... it'd absolutely look like Whitebeard!