Maybe I want a gun the shoots ice? Maybe I want to trade knocks for more damage? Maybe I just want the 'Kirby Krackle' for my power armor?
I think it would be great if power replacers made a comeback and were improved. A bit like the auras and the item slots, add an extra slot on our gear that we can slot power replacers into to change how our powers look and work.
Purely cosmetic ones can used in the aura slots (maybe a new one) and the quality of you gear might affect how well it works. So that, for instance, native ice powers have more effective procs at first but by end-game you are better modded than they are unenhanced.
Speaking of which, what if you're happy with the normal powers? Those slots can be used to enhance you existing powers without changing them.