I think I'm the only one, but I never use the visualizations. Maybe they just don't go with the music I listen to. They kind of give me a headache.
But as a music lover & kind of insane collector I'd be all over something that embellishes music the way TV & Movies does... Starting with Fanart, but getting into studio and label logos, etc. Wow. Basically making the info screen more like the actual liner notes.
And something like cycling through fanart for the band that you're browsing or playing would be great, too. To have your now playing list, your current album art, and the backdrop showing a slideshow specific to the song that is playing. Cool.
Some of that might be XBMC mods or additions, I'm not sure. Support for .nfo's in music would be a great start. (That doesn't exist yet, right?)
That being said I'm very happy with what we've got. And the additions I'm talking about would be a ton of work on the user end to fill in on large collections. Working with hundreds of movies nfo's is hard enough - tens of thousands of songs on many thousands of CDs and records... that's a monster.