I know this might be an unpopular idea, but I'd love if there was a way to farm Rep with the Horde as an Alliance to gain a standing with them where you are at least neutral and could visit cities from the other factions. Might be too big a project and of course didn't have to be as easy as it is to become a diplomate where you can speak with the other faction, but if you could earn it with each individual faction (thunderbluff, orgrimmar and undercity for example) so that you were neutral and could still attack NPC's in cities (which could throw you right back to hated).
Always dreamt of something like this, maybe it's just me :P