Rezist has an interesting idea on the forums. We've already seen lots of ingame marriages happen, just for fun, but what if the marriage process ingame was formalized? Right now, you can buy a tux and a dress and pretend to get married, but what if the tabard vendor also sold a marriage license, so that you could formally connect two characters in marriage? Of course, it would have to actually mean something-- maybe the two characters could share a bank, or even get some kind of combat bonus when they were grouped up or near each other. With a Valentine's Day patch coming up in a few months, it might be a fun twist to put in there.
On the other hand, marriage itself brings up its own set of problems. Would marriage between people of the same sex be OK? Odds are that's a bomb that Blizzard doesn't want to deal with. And how would divorce work? A formalized ingame marriage could be as messy as marriage in real life if not dealt with properly.
Later in the thread, Nethaera shows up to turn the conversation from RPing in general, and I agree with her-- we're already playing characters here, and there's nothing wrong with doing something cute like this to let two people playing the game show a little affection for one another. But unfortunately, odds are that this is a little too messy an issue for Blizzard to put much work into.