First i’d like to say i’m perfectly happy the game has a finite progression, the D3 Paragon system was a mess.
Still i think it would be nice if once you’re level 100 the xp you’d get would convert to gold (at a ratio that would make sense) and maybe give mobs chances to drop small amounts of obols.
Once you’re 100 the only thing left is optimizing/trying builds to try to tackle the highest NMD and Uber Lilith which requires very specific stat combos who are both hard to fish for and insanely costly to reroll for. Making that process a bit smoother as a reward for hitting max level would be a nice way to keep people engaged past that point (as very few people do in my experience, most people i know just hit level 100 and immediately quit or rerolled) without power-creeping the game.