Steampunk, SCA, and Renfair all have a costuming element, yes? So approaching these topics from that angle might make it easier to fit them in Fanlore's current structure.

I don't think "mainstream" music or sports fandom (the non-RPF aspects, I assume?) are within the scope of Fanlore, but I'm not familiar with them. The question to ask is: are they producing creative works? I gather there's some zine publishing happening in punk, but a "subculture" isn't necessarily the same as a "fandom." Hard to say where the line is.

Civil War reinactments are probably not within the scope of Fanlore either--reenactors don't think of themselves as fans, do they?

I know Fanlore would be happy to have more information about gaming in general, but I don't know enough about it to advise you on specifics. There are already some media fandom livejournal RPGs documented on Fanlore, but they can be read by non-players like fanfiction. Come to think of it, I'd like to read a page that compares Livejournal RPGs with other types....

If this helps, the What_Fanlore_is_not page says that Fanlore is an encyclopedia of fannish culture, as opposed to an encyclopedia of pop culture.