Personally I believe that it is better to reach max level earlier so it can be properly enjoyed, than to reach max level just at the end and barely get to enjoy it. With that said, Divinity Original Sin 2 did leveling phenomenally. There is a soft level cap of 20 which can be reached easily before the end of the game, but there is no XP cap whatsoever... so the player can reach even level 21 legitimately if they truly put their mind to it.
For Baldur's Gate 3 I think it would be better if there weren't any XP limit, but any level up obtained after level 12 would just give the player an option to multi-class into a different level 1 class. No stat increase nor level increase whatsoever of the existing classes, just level 1 of another class to enjoy their bonuses. That way it would retain the thrill of gaining XP without breaking the balance.
I don't think that would be a good idea at all. First, I would not want to be forced out of my primary class. Just imagine if your DM did that.
Secondly, this works a lot better for some builds than for other. A fighter could get a lot of extra power from multiclassing into Paladin for Smite. A character with Smite + 3 attacks per turn + Action Surge is definitely as balance breaking as far as things go in BG 3.
A primary caster like a Wizard or Sorcerer doesn't have that many good options. You would almost feel forced to go into Fighter for Action Surge, since it's the best option by far (assuming no more spell progression).
