I feel this is a great idea, Tore. I recommend bimonthly themes, but that's just because I think I wouldn't have enough time to give a weekly theme the attention I'd like before the next week's theme arrived. Other people might really like weekly themes.
I notice you don't have the Canterbury scene on your list!
I know it isn't really a "genre" but a combination of avantgarde, progressive rock, and jazz...still, why not add that! Maybe it can serve as an example of a mish-mash genre.
I like the fact that "easy listening" is on the list. I hadn't expected that! Heh heh...the thought of MusicBanterites commenting heavily on Muzak will be a thing I'd like to see.
Does MusicBanter already have set times when people listen to a list of songs together while in the shoutbox, so that they listen at the same time? Another forum I'm part of (cough cough - Gaa! It feels like betrayal!!) does that, and the times I've been able to join in were lots of fun. If MB doesn't have that sort of thing, it might be nice to have a list of youtube songs or downloads representing each weekly or biweekly theme, and then set a couple times in the week (to account for different time zones and schedules) when people who are interested can meet in the shoutbox to listen together to the same songs at the same time.