For me, it's all quality of life stuff. There are quest mods that people seem to like but I haven't tried any. The mods I use all all stuff like infinite stamina horse running. Zero inventory weight limits. Full experience from every quest regardless of how overleveled you are. No weapon degrading. No fall damage so I can jump down from heights in novagrad. That basic stuff I grab off steam workshop so it's always ready for my new game+ saves where I have an insane inventory. Nexus when I feel like trying the cooler stuff

Here's the quest mods sorted by endorsements on nexus

https://www.nexusmods.com/games/witcher3/mods?categoryName=Quests+and+Adventures&sort=endorsements

I hope the new expansion sparks a new mod era for witcher 3. Would love witcher 3 as the base game for quest mods rather than skyrim. Witcher 3 graphics just so nice to look at

Getting rid of item weight, more fast travel points, question marks on map only showing when near, no fall damage, restored cut content, naked geralt, hiding ununsed ui elements, better mini map, unlimited perks, health regen on higher difficulties, and most importantly to me auto loot. Lots of little small things and annoyance fixes.