Personally I would really love a Definitive Edition. Was pretty much the only thing I was excitingly hoping for to come.
Quality of life additions (like more intuitive UI, better inventory management, more fluid controls, more keyboard shortcut combinations to reduce the amounts of clicking etc)
Improvements to existing content (like implementing reactions where the story currently lacks them, ensuring quests feel more dynamic when their sequences aren't followed to the letter, more fluid gameplay and overall control, better appliance system of dyes (it's quite wonky), all equipment slots having a toggle to visually hide them - especially capes etc)
Restored cut content that underwent minor revisions (by that I mean scenes that didn't make the cut but with minor revisions could make it back without an issue. Larian's current MO is to just entirely scrap a scene even though 90% of it perfectly fits the lore)
Additional minor content (such as adding a bit more content to certain companions to make them more fleshed out; primarily Minthara, Halsin, Wyll and slightly Karlach)
Optimizations and bug fixes (like making the torch slot act properly so it stops shuffling abilities all over the place, ensuring all gameplay mechanics work properly, ensuring all multiclass mechanics function properly with certain combinations, making heavily scripted areas not act so stubbornly scripted - like the Moonrise Prison where its Warden and guards break universally established gameplay rules etc)
Basically to cover up most of the story holes, fill up content where it's lacking and improve the overall gameplay experience. There's a lot of room for improvement that could make the experience drastically better.
