The most realistic guess is level 12 in my honest opinion ...
I mean we were told that original max level has ben 10 ... later Swen told us it would be some more and they dont yet know how much more ... so "at least 11" would be logical to presume ...
Now when you look at features table, all every class gets on level 12 is just another Ability score incerase ... and some gets additional resources (like Barbarian who gets 5th rage) ... that would mean level 12 means litteraly no additional work ...
So i dare to say there is nothing holding Larian back to implement it aswell.
Look futher tho, and you see that on level 13 many casters get acess to another spell level ... namely Wizard gets acess to spells level 7 ... that alone can be reason for not implementing the level. :-/
Bcs it means whole bunch of final game spells you have to implement, yet another reballancing the game bcs there are quite huge steppes between spell levels in matter of power, and simmilar stuff ...
In short: Lot of work.
Now, if you remember when Druid was introduced Swen litteraly told us that Nick wanted to include Dinosaurs in the game at least as Wildshapes ... and Swen borbided it, since that would mean a lot of additional work, i believe his exact wording was "we would like to also ship this game someday" or something simmilar, dont quote me here.
So if they had to exclude some low-level features ... it seems quite logical, that top-level ones gets the same threatment.
