Since I never liked Keiko, it would have been interesting to see how a Miles/Kira pairing could have turned out. The best romances start out as friendships, so it wouldn't have been a "one day _____ realized that the love of his/her life was right under his/her nose all the time!..."
I get the impression that Jadzia would have had sex with just about anything, so... why not pair her with a Horta?
As for same-sex... definitely Garak and Bashir. Garak was always semi-flirting with Bashir anyway (whether he was serious or not), and Bashir was fascinated by his Cardassian friend. So you never know...
I read a really excellent fanfic of a Seven/Tom pairing. It was Tom
who realized that Seven wasn't ready for a sexual relationship, being mentally still about 6 years old and not really understanding the full scope of what she was asking him to do. Still, that's a relationship that could have progressed from friendship to love over the years, as both of them matured as people.