Auto-retagging would be a VERY bad thing.
But having it be a something that's applied onto the track before it loads, that's a different story -- it already does this with your character picking anyway. There's various ways it could be done, none of them affecting the files or requireing any editing outside the game.
I've said before, as fun as the game is, it's really very sloppily done. Sure it's only done by one person for the most part, but even so there's a lot of polish that could be added to simply make it that much more.