There are many different types of end-game goals in Neopets
1. Pet-related
As the previous person mentioned, you could going forward to trading for a UC that you really want. Or buying items via NP or NC (real cash) for your customization. Or it could even mean training up your pet's stats and buying strong equipment for them to fight in the Battledome.
2. Collection-related
It could be stamps, trophies, avatars, galleries. Anything that you enjoy that makes you happy would be part of the collection-related. Things like trophies don't cost money but more of your time and effort into playing and getting good at them. Or if you cheat, then it still takes time to get gold trophies on everything.
3. Money-related
Having maximum amount of neopoints (>2b) are some end-game goals for many as well. If you're not into grinding your pet, or collecting anything, then having the max amount of neopoints could be an end game goal.
Or it could even be a mix and match of a few things. It's really up to what makes you happy and how you play the game! You can do anything, and you're not limited to one certain goal, that's why it's so appealing to me