I doubt this will ever happen, but what about a service where players have a NPC leveling up their pets for money? But it will of course cost gold. Like the daycare in Pokemon. But the price will increase by 25% every level. Like leveling a level 0 to level 1 is 250 gold, then, 500 for level 3, and so on. SO yeah. IN the end it would be very pricey.

This is neat idea, though I'm not sure if this needs to be implemented yet. The max level you can achieve right now is lvl 60, which isn't all that hard to reach (especially, if you do a lot of missions). It might also make it too easy to level your pets for the Arena, taking a lot of the challenge of this game aspect away. Perhaps it would work best if there was a leveling cap on it, say lvl 20? That way you would still have to do some work leveling your pets on your own.

Another option would be to have multiple ones, each with their own lvl cap, in each of the cities. For example:
Esior's Haven: lvl 5 cap
Sanctuary of Saerielle: lvl 10
Ethernia: lvl 15

Or something similar.

You do have the experience orbs that can be found in random drops, or from those selling them in trade.

I know I know. XD I"m already at CAP, but just thought this"d be an interesting idea.

Not an idea that needs to be used. It really isn't that hard to level a pet through missions, experience orbs, or just battling. There really isn't a reason to level a pet beyond arena/questing/missions/dungeons, and I think that it's just better to encourage a "do it yourself" mentality if you want to level up.