During Rainbow Man's end credit scene, you see him hanging out with Roll, possibly a date, which can be a reference to the Megas' song "Don't Mess with Magnetman".
Mr. Shin'iri, an enemy found in Jet Man's stage. He's a tourist carrying a suitcase that runs quickly as if he's late for a flight. When he approaches Megaman, his suitcase explodes into a swarm of Pipis for no apparent reason and he runs even faster in panic. It's as silly as it sounds.
Comet Woman's Mettaurs. They act normally, until they whip out their BFG and fire their Ray Gun until you explode.Frickin' Laser Beams.
In Glue Man's stage, they're actually several Mets stacked atop one another.