late game seem to be designed around the level cap being achieved (...) Either way, perhaps the solution is an additional skill level?
Disagreed. There aren't a huge gap between lv 11 and 12. 4 to 5 has a much bigger gap as well 10 to 11.
About aditional level, Larian would have to add all tier 7 arcane and divine spells as well some class features and re do some encounters.
Not mentioning, 5e suffers from the same problem which Obivion suffers ie - hp grows up much more than damage. At high levels, things become too bloated.
