I really love this game and the fact that devs still adding content is awesome, but with more content goes more exp for character. It leads to hitting 70-80 lvl just playing first playtrough, that leaves small window for more challenging ones. And I yet didnt mention that important part of this game is farming - combat visuals and sound are satisfying and loot is interesting - it is also very rewarding activity that I find hard to resist.
So I made mod that decreases EXP from kills, but may be you cold add some kind of slider in a menu that can adjust exp rate downwards? Not necessary to zero, may be there should be lower cap like 33%, but from all sources including quests, notes, etc. Would be cool both for farming and for complete playtroughs.