What we know from the current Rank 10 content about Rank 11:
Rank 11 will feature the last Revelation to find. It will probably be quite similar to what we’ve seen so far, new maps, new quests,new items.
Judging from the ingame lore, it will also be the last Rank before the game changes completely. Laimas Revelation of the Astral Tower revealed that we will face off against Giltine and meet Laima in person afterwards, it will be “the end and the beginning of everything” as stated, changing the game mode from learning about our destiny as Revelators to offensive onslaught against the demon kings and other evil gods/goddesses from Rank 12 onwards (if there will be Rank 12).
Now about the game mechanics/plan changes:
As IMC stated before,they plan to separate levelcap updates and Rank updates, yet they didn’t so far for Rank 10. Maybe it hints at the possible endcap of Rank 11 considering the Rank is ± consistent with the Revelation-updates.
So, while we will probably continue to level beyond 420+ with regular updates, the Rank will possibly no longer increase.
I think this is the point where IMC will try to approach the older Classes with their “Master Circles” as an alternative to the current Rank-up system, because it’s easier to balance while it limits the possible build choices.
Another possibility is that it will happen after Rank 12 [which will probably be the upmost limit,otherwise you’d be able to stack two R8 Classes at C3 on top of each other…], but that’s very uncertain, given the current status of the game.
As stated in the current Popolion Post, they will also focus on “endgame content” [which might be a little early unless they plan to bring Rank 11 early (maybe next February? It should be fast now that there are no new skills that need to be developed for Classes) and weave it into the lore about fighting Giltine and/or the real Demon Lords].
From the current gameplay and mechanics to judge from, I think it will be a “hard” and probably difficult to fit the “endgame content” category. It will probably scale with level,too and adjust difficulty and rewards accordingly, as this is the only option to keep endgame content viable as “endgame content” while the game advances.
Who knows, maybe we will see something very cool like a server-wide cooperation mode to trigger/beat it(partially or certain specific group contents outside of the regular party), but that would be too far into speculation at this point.
All in all these are the points I believe we will see at Rank 11 and onwards.