Basic idea is simple, you can take your charmed pet to an NPC. That NPC will remove your pet and convert it into an inventory item. (Attribute of pet, type of pet, maybe the name?, level. Example: Oink, Elder Warthog - 17. Don't really need the name though, would be nifty, allow rename of course.) Also, don't need to be able to bring them out in towns, would be nice. Talk to an NPC to release the pet again.
Now you can sell your pet that you raised from a little baby. Or that black widow is simpler to obtain. Your pvp char can finally use something different! Maybe you just want a different pet for different situations and not have to re-train them over and over. Gotta get em all?

Maybe even transfer them to GW2. Another thing that would be nice is instead of talking to the NPC over and over, a slot could be used for whichever pet you want at the time (Just drag and drop the pet in, acts like an inventory item slot. Still need to get the pet 'converted' first. New PvP char pets can't be 'converted', prevent free pets.). Maybe drag and drop on to the Charm Animal skill, which when not in use can 'house' your pet so as not to take up an inventory slot.
Main ideas (counting only a /signed or /unnotsigned - July 12 #20):
1. Retain pets in some form for later use. /s - 2 /u - 0
2. Possibly allow trade/sale of pets. /s - 0 /u - 7
Also some way to unlock or transfer pets to a PvP char, which could be either.