Alright, basically every new (non-clone) character has a very unique trait or feature. (robin=tomes, palutena=customs, pacman=summoning/retro transform, little mac=ground based, etc)
What would King K. Rool's trait/feature be?
Perhaps for a small amount of time he can "enrage" with his down-b, go crazy and do lots more damage and knockback and suffer less or no hitstun/knockback (but take extra damage). You can use it once per stock, only above 80% and it lasts for 1 second for every 10 damage rool has taken, with a max of 20 seconds, and damage taken while enraged increases it (max remains 20 seconds total). After enraging ends, k. rool faints to the ground/goes into freefall and lays there for several seconds, the shockwave is very damaging. After a moment k. rool will jump up and laugh, ready to fight again.