^ The Riley thing would have been great!
I think the only reason they didn't let it get that far was the trouble of making that spell and its effects last for more than one episode (one for the spell, one for the wedding and short lived marriage, and one for breaking the spell and ending the marriage) as well as for not wanting to write out the complexities of how to go about marrying a 125-year-old undead guy with no identity and no last name.
All of that said, I would have been willing to suspend my disbelief for a wacky Vegas insta-wedding with, like, a vampire Elvis or something presiding. Hehehe. Ooh - or maybe a cheestastic goth/vampire themed wedding with, like, coffins and stuff. Bahaha. Then the vampire spin on marriage would have made me giggle, I'm sure. Just imagine the chore wars:
Buffy: Why don't you ever take the garbage out?
Spike: Because I'd explode and die, you stupid bint!
(Replace with better dialogue).
Finally, the breakup would have been funny. It would have upped the ante on the revulsion that we saw at the end of Something Blue -- and also, after a week or two of marriage, they would have had permanent blackmail on one another.
All in all, though, I think they did a good job of touching on all of those issues in Something Blue. Apart from the being insanely funny factor, making an arc of it wouldn't really have moved the season's story forward.