Janice Rand and Sulu. They had great chemistry in the Man Trap and I think it would have rounded out both characters.
Kim and the twins. Not in a pervy way but they were astrophysicists and the only scientist Voyager had apart from Janeway was an astrobiologist.
Ensign Ro and everyone.
Janeway and Q. What a lovely couple.
Some of these interactions actually happened, though. I think Lore and Armus would be an interesting pairing, also.