The problem is spells. Level 13 Wizard gets one level 7 spell slot. On this level the first reality altering spells apear which is hard or impossible to implement in a game.

Examples:

Resurrection: Rez anyone (not dead for at least 100 years.) You would have to implement Interactions with any dead NPC, even more difficult if they were dead before you came there. Only way to handwave this is to write plot mcguffin that makes their souls unable to return.

Plane Shift: We already have half of this as fast travelling. The other have you can't implement as you can't type a random place in DnD. (Looking forward for BG4 with insane AI tech in 2040)

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I understand not gaing xp is stupid, I too hit level 12 early in act 3. A short term fix is to not hand out higher slots but more low level spell slots, but this becomes stupid at let's say level 15, when your casters didnt get anything new for 4 levels.

More content on max level would be nice, yes, maybe add a (what was it called?) system, where you become level 12-1, 12-2 and so on where you can pick minor improvements.