I search now wiki here and there and i make a bet that his future rank up's will be those:
Vajrayaksa overlord -> Maitreya ______ (Avatar for example)
Maitreya is buddhativas that will be next in charge of our world. Yep in buddhism there are attendant who will work when rest either go for buddhahood or wait for their turn. More his pretty much assosaited with protection and benelovence. Some one who will come intro world to bring it intro transcedence.
Maitreya ______ -> Amoghasiddhi _______ (Deva for example)
Amoghasiddhi is buddha that is associated with the accomplishment of the Buddhist path and of the destruction of the poison of envy. Also he's name means noble star.
I base my prediction on the links in japanesse buddhist pantheon.
Everything in that worlds i devided intro 7 category so i don't belive that he can rank up as a monster more. Beside in the buddist pantheon there are no more level of enlighment/ascension. But maybe he will become demi-god? Who knows. So summarising:
1. Goblin
2. Hobgoblin
3. Ogre
4. Apostle Lord
5. Vajrayaksa Overlord
6. Maitreya ______
7. Amoghasiddhi _______