for me standard pet sites have been done - and you really need a new angle
one of the best ideas for a petsite i have seen was creature breeder - where all the creatures were developed by genetic mating - i thought the idea of pets where the site developers don't know how the pets will end up
the problem with that site was i felt it was way too focused on just farming loads of pets, there was nothing to really pull a community together other than to share and breed cool pets (that died after a few weeks)
i thought a site were you have only 3-4 pets, and it was less 'breeding' and more exchanging - this happened by meeting other pets and either having fun with them where the genetic algos would choose one path, or you could battle other pets and this would cause other genetic algos to take place (i was thinking the fun algos would be more for learning, happyness, look, feel, etc... and the battle algos would be for strength, power, activities, different look/feel stuff)
there could still be breeding as that would be pretty similar to a meeting.
you could trade/sell cool pets, there could also be items that allow you to transform your pets without the need of other pets - to add things into the genetic makeup or the pet like fluffyness.
maybe you can make it a bit more complicated by actually making 'items' which can be obtained to select the type of genetic you want to transfer into other pets so that they can have that attribute as well. Something like transfering claws to fishes that kind of thing.
Looks pretty cool

, but calling the game money "Mezos" seems kinda iffy

maybe 'candionz' or 'dionz' might sound a bit cooler for game money ?
lerbt ?
kertz?
yorkre?