Rinny (Little Cute Thing) vs Flippy/Fliqpy (Happy Tree Friends)
Both are bears that look innocent at first glance, but really sadistic monsters on the inside.
Look, I just got into Little Cute Thing just yesterday and only saw four videos. This was the first MU that fits for Rinny that popped into my mind (I thought of Eric Cartman for Rinny, and thought of the story happening throughout the fight, but that's just short, sadistic assholes).
Would DEATH BATTLE- you know, why the fuck do I do this when the answer is always obvious?
Omni-Man vs Zim
Both are aliens that have be sent to Earth on a mission to conquer it (Nolan: Lower Earth's defenses, Zim: Get the fuck out of here, you're annoying, Zim!) by an organization infamous for conquering planets (The Vilgermites and the Irken Armada).
The contrast is that Nolan was sent there on a serious mission, while the Tallests only Zim to get out of their non-existent hair.
Nolan is cold and no nonsense, while Zim is crazy and the king of large hams.
Just imagine the interaction of these two.
... I don't got a third MU idea, sorry.