The Witcher games have been some of the best story-driven RPGs around of late, with an in-depth story focused on Geralt of Rivia. Things are being expanded for The Witcher 3: Wild Hunt, with a 50-hour main story (plus another 50 hours or more from side quests) and a game world 20% larger than The Elder Scrolls V: Skyrim. The Witcher games have always been single player, but that may change for The Witcher 3, as CD Projekt RED is "thinking about" multiplayer.
In a recent and rather lengthy interview, CDP's managing director, Adam Badowski, would only say the following about multiplayer: "We're thinking about something but I cannot explain it now. You can expect some information later on."
Badowski did say it shouldn't be similar to the Dark Arena from The Witcher 2, but again he avoided outright confirming it. One thing he did announce is there are no MMO plans for the series since the "story is not perfect" for one. That should make many people breathe a sigh of relief, at least.
A multiplayer mode in The Witcher 3 could be very interesting depending on how CDP pulls it off. Co-op would probably make the most sense, but some could argue for a deathmatch-style between the different factions. Whatever the case may be, nothing definitive is being said one way or the other just yet.
There's plenty more discussed in the interview besides multiplayer, so head on over to the source and give it a read.