There are some hints that this could actually happen, considering if you follow the main storyline of the Witcher 3, and hear about the dialogue when talking to Ciri at one point, you will hear something amazing that could actually make this happen, and it will be wonderful to see this game somehow in the future, become a game similar to Star Ocean setting wise. You think this might actually happen before or after Cyberpunk 2077 releases?

Perhaps an easter egg rather than a proper crossover.

It can happen, and this will make the witcher settings turned into a bit like Star Ocean setting, a convergence of a mix of both fantasy and sci-fi, and we already seen alot of certain modern imitation formed into magical way since witcher 2. I won't be surprised to experience something very interesting to spice the lore up on witcher 3, expectantly if another sphere that is linked to garalts world could be highly advanced stumbled across his world by mere coincidence.

Cyberpunk takes place in a licensed original setting. Witcher takes place in a licensed original setting. CD Project owns neither of these settings. The conversation with Ciri was certainly a vague reference to Cyberpunk, but the settings mixing in any fashion is highly unlikely, purely due to limitations imposed on CDP by owners of those IP.