I don't think we'll ever go beyond level 25. At some point we have to stop in level. Instead what will happen next is Epic and Legendary battle-stones. Hopefully they will not be as commonplace as the Flawless Battlestones, which anyone who is level 100 with a Pet Menagerie 3 and a few decent level 25 pets can get at least 2 battle-stones a day.
I would like to think that if Epic and Legendary upgrades become available that we'd have to work hard to make them happen.