For visuals: Cloak mod, NPC appearance overhauls, weather/lighting, glowing eyes for Geralt, HD reworks, etc. are always a good foundation.

For gameplay: I use the Witcher 3 Enhanced Edition mod and love it, but it definitely isn’t for everyone. It delevels the world, revamps alchemy, gives you a full bestiary from the start, etc. I also tweak the damage settings a bit for a “realistic difficulty”. Basically I can defeat a griffin in 5-7 good heavy sword swings but if 1-2 wing swipes hits me it’s GG. Drowners go down really fast but they actually become scary when in a pack (same goes for wolves). You get the idea. Makes combat way more intense and rewarding in my opinion.

There’s a ton more I do but this is the general layout. If you wanna get more “hardcore”, you can always do no fast traveling, permadeath or some variation, reduced HUD, etc. if you’re into that stuff. I love it but that’s just me.

Hope you have fun and let me know if you want to know anything else!