I definitely don't want a hard cap to exp gain, where I gain zero XP from additional monsters/quests. That would feel like I've been punished for fully exploring the world … If I had to choose, I'd want a soft cap where exp gain is decreased
I agree that we shouldn’t feel punished for fully exploring, but don’t want to feel overly incentivised/coerced by game balance into killing every enemy or completing every single quest where that doesn’t fit with my roleplay either, or indeed into using XP-maximising exploits.
I do like your idea for subtly and progressively reducing the rewards for XP harvesting, if that turns out to be needed to amplify the effect of what increasing amounts of XP for higher levels will already do to keep players at roughly the same level through the game. It would certainly be a lot less intrusive and dispiriting than some players hitting a hard cap way too early.
I’m still perfectly fine with there being a hard cap at some point though, as long as it’s sensibly balanced. I still don’t want Larian spending time developing and testing player levels that are only likely to be achieved if someone goes out of their way specifically to maximise XP rather than play the game as intended (uncomfortable as I am with the implication that there’s a right and wrong way to play!). And actually, though this isn’t a big thing for me, I do quite like getting to a maximum level and feeling I’ve made my character as powerful as they can be for the endgame, as long as I don’t hit that point too early and as long as it takes a reasonable level of work and side exploration to get there.