newest pet ideas can be found towards the end of the post.
1. highest vanity pet tier achievement (collect 100, 125, 135, w/e the number may be): "Animal Handler" - rewards = title (Animal Handler *Insert Name*) plus a leash item that allows you to have more than one vanity pet out at the same time.
i've always wanted to be able to have a few pets out at once. sometimes it's hard to choose just one to play with, especially once your collection gets up into the triple digits.
2. companion pet window: sort by pet type, such as "the bestiary" over at warcraftpets.com. or a feature that allows you to search and find your pet by name. (example would be the trade skill tab search feature.)
you've got your mammals, birds, mythical, mechanical, etc. once you get up to 100+ pets, it starts to become a chore to scroll through the entire list even if it's listed alphabetically.
3. pet biscuits that make your pet itty-bitty.
there's already a consumable to make your vanity pet bigger, so why not the other way around? i'd love to see a tiny version of an already miniature version of various WoW creatures. but maybe that's just me..
4. adding another pet-rewarded quest and/or quest chain completable by both alliance and horde.
some examples would be: the sprite darter quest line, miniwing, mechanical chicken, smolderweb hatchling, worg pup, etc. the best example would have to be the sprite darter chain, though. the starting quest was off the beaten path that fewer knew or wanted to walk. it made obtaining this pet that much more special!
5. gnomish (or goblin?) companion pet "grooming" kit (will need to think up a different name, since a "grooming kit" already exists).
ever wanted to spiff up your pet? maybe turn that giant sewer rat pink to match your tabard? introducing the new grooming kit! customize your vanity pet to your heart's desire! (use with caution, some limitations apply, do not use on a daily basis as frequent dying of fur, feather or scales may be bad for your pet's health.)
6. gnomish (or goblin?) companion pet creation device.
like playing doctor? how about genetic scientist? select qualities and attributes from already existing pets to create your very own, unique vanity pet! chance of failure: this device has a 99.9% chance to spawn a mutated creature or mutagen. use with caution! no animals are harmed in the creation or attempted creation of vanity pets.
7. one time use, pet cloning device.
it would clone a pet that you've already learned, so you can give one to a friend :) one time use only, though. and the item itself would probably be difficult to obtain. if it were unlimited and easy to acquire, think of how badly the AH would be flooded with "rarer" pets.
8. companion pet "bling" collar.
do you enjoy that just-bathed pet shine but don't have the time to constantly rewash your pet? purchase a diamond studded collar and your pet will shine all the time! one size fits all, use it for every pet you own! note: remember to remove the collar before putting your pet back into its cage as it may be a choking hazard.
9. PetScore!
modeled after GearScore, this addon would tell you how many vanity pets the targeted player has, and what "pet ranking" they are on your server! now you'll never have to group with people with less (or more?) pets than you! :P
10. trainable pet tricks.
much like the book of glyph mastery teaches an inscriptionist how to craft a new random glyph, a "book of companion tricks" would teach a vanity pet owner how to make your companions do tricks! you have to own a vanity pet (ie: learned the summon spell) in order to use the book. also, the tricks would be limited, because they would need to apply to all types and forms of vanity pets. they would have to be basic and simple, ones that all pets have the ability and animation for. i can't imagine a disgusting oozling wagging its tail for example..
more to come!