This is just my own speculation but I really believe that they might be working on a remake of The Witcher 1. In a financial report, they stated that they plan to release two AAA games by 2021. One is obviously C2077 while the other game is unknown. I am doubtful that CDPR has the capacity to work on two AAA projects concurrently unless the other project already has some sort of base to work from.
The other project is apparently Cyberpunk-related, a smaller-scaled RPG utilising CP2077 tech. But this wasn't properly confirmed, though.
That said, I would love to see The Witcher 1 remake. But the more I think about it, this would require serious amount of resources. The Witcher 1 was developed on a different tech, PC-oriented engine with constraints that made it a vastly different experience than The Witcher 2 and The Witcher 3.
If they wanted to make it more modern, they would need to reimagine the whole game - combat, exploration, how to translate world from module-based into more open environment, fix quest design (TW1 had a lot of fetch-quests), issues with pacing and backtracking, or even do something about that horrible romance cards.
But with a right partner, they could definitely do it. They actually tried to do a semi-remake of The Witcher 1 in the past on PS3 and Xbox 360. It was called The Witcher: Rise of the White Wolf, but it was cancelled because of financial troubles and some issues with a developer who was tasked to make this project.