Despite all elite four have pokemon with mega forms(counting mega steeling for bruno), they are not from their signature pokemon(excetp for agatha)
So, a mega lapras, mega machamp and mega dragonite.
Agree on mega weasing and arbok for team rocket.
Also there is pokemon that could receive type change, golduck needs a psychic type.
Id add butterfree, dewgong, dodrio, hypno, kingler