You defeat an entire team of gods in the the game, who are you going to battle in the DLC?
There's a mod for extending levels.
If "becoming too powerful" was a genuine concern for the designers, we wouldn't have an entire skilltree of "Mindflayer power-ups" and a whole collection of house rules and/or incorrect implementations making our characters far more powerful that they are supposed to be at level 12.
This, honestly. I don't buy the idea that the level 12 cap is because things "just get too powerful" at higher levels. They effectively make you as powerful as characters 3-4 levels above 12 anyway, by endgame, with absurd items, additional powers, and unbalanced rule interpretations.
There IS the problem that spells at higher levels tend to be more broad in their interpretation and may be harder to faithfully implement....except for the fact that they're clearly okay with a "video game" implementation of spells that work. It's not like they implemented all spells from levels 1-6, or that the ones they did implement were done with total flexibility and faithfulness to their tabletop versions. "Wish" could easily be implemented: Give a handful of one-time options for some neat permanent boons (you can even implement the 'might cause you to not be able to cast wish ever again' rule), and then also have its standard use (any other spell, up to 8th level, in the game from any spell list, upcast to 8th level.)
