Halk Hogan has made a bit of a name for creating HD texture packs for Witcher 3. I am not in the habit of modding game textures after a certain scroll game became too much of a burden with them. Having to trouble shoot compatibility and crash issues can really ruin a afternoon or weekend. Thankfully the packs from Halk do not usually have that issue.
I tried one back around last fall and just finished doing another 4k play through with it recently. Was completely blown away. Could not tell you how many times I would just stop somewhere to look around. Some people joke that Halk is turning Witcher 3 into the game that was originally demoed at shows before release. After over 400 hours with this game it is unlikely I will play again anytime soon but for those who have not experienced one of these mods another one was released about a month ago. It too looks even more impressive than the one I had used.
Something worth noting is that Halk pulls this off without major performance losses or majorly increasing V-Ram so there is some optimizing at work. Not sure how they do it but they have it down to an art form. It has not been announced when the next one will be released but here is a link for the one that came out on August 5th.
Enjoy the video showcasing the next one. I recommend heading over to his YT page just to read the praise. It is a challenge to find complaints. Feel free to comment in the forums about your experiences with them.