I picked "For a short period of time in order to supplement the story. (like in W2)", but such segments do not need to be short. I simply want them to show events and places significant to a story, instead of a character just telling Geralt about it. I want it to be only in separate, unique locations. I would prefer such segments to be longer than in TW2 and to include dialogues, fights, and some minimal exploring, but not questing, looting and leveling. In short, just build up on TW2 experience, and make them more significant.
I do not want any switching between Geralt and his companions, and I do not want any quests done by the second character. I prefer to have it only as a story-telling device, and not as a part of game mechanics.