In a recent Developer Q&A many topics were touched on, including Pet Battles. Here are the three Pet Battle questions and developer responses. [source]
Q: Currently Pet Battle are only between the same faction. Is it possible we'll allow cross-faction pet duels? Will we have a Spectator Mode for pet battles?
A: There are currently no plans to make Pet Battles cross-faction. There isn’t really a need to build in that functionality either, provided enough players are participating that the queue times aren’t long.
Q: Do we consider adding further music tracks to the pet battles? The current music playing during the battles gets a bit old after a while.
A: There is an option to turn off pet battle music specifically if you get sick of it. We don't have any plans at the moment to expand upon the existing pet battle music. We did shuffle 10 tracks into different playlists and added some logic to the way they play back though. Specifically, PvP battles have the more driving pieces, and the trainer/boss battles have longer tracks that are less likely to repeat during a fight. PvE (wild) has overlap with both of these, but is weighted more with the less intense music. It's minor, be we feel this improves the experience by tying each music piece more consistently to different types of battles.
Q: There are rumours about several UI improvements and other features being added in patch 5.1 for pet battles. Anything you can share on this?
A: Just as an example of something awesome we plan to do with the pets that drop from old raid bosses you can share, if you collect all of them you get an achievement that unlocks Mr. Bigglesworth from Naxxramas. To distinguish him from the standard Siamese cat model, we gave him some sweet frozen whiskers and a frost path behind his footsteps. He's Undead.
For a full transcript of the Q&A, click here.