I think Iron Man and C - 3PO would make a great pairing. Iron Man could upgrade C - 3PO's systems, and C - 3PO could provide Iron Man with valuable information about different cultures and languages in the Star Wars universe. They're both tech - savvy characters, and their interactions would be full of witty banter.
Thor and Chewbacca would be a hilarious and powerful pairing. Thor's godly strength and Chewbacca's brute force would make them a force to be reckoned with. Thor could also share his knowledge of magic with Chewbacca, while Chewbacca could teach Thor a thing or two about loyalty and friendship within a group like the Rebels or the Avengers.
Black Widow and Ahsoka Tano would be an amazing pair. They are both strong female characters with great combat skills. Black Widow could learn some lightsaber combat from Ahsoka, and Ahsoka could pick up some stealth and espionage techniques from Black Widow. They could go on secret missions together in a crossover story.