I doubt it's Fallout 4 for Switch, just because it wouldn't make much sense for them to release it so close to Skyrim. I also feel like they would have announced it alongside the PS4/XBO re-releases if it was happening.
My money's on a Switch port of DOOM. As I've mentioned a few times before, it's actually a good candidate for a relatively quick Switch port, as it's got a Vulkan renderer (Switch's NVN is based on Vulkan) and performs pretty well on low-spec PCs. It would also get quite a bit of attention for running on a portable, and would have a different enough audience from Skyrim for Bethesda to not have to worry too much about releasing them close together.
Rockstar just today announced a new release on four platforms for this year. I don't see Bethesda announcing a totally new game, but a port of an existing title to a new platform doesn't need a whole lot of time between announcement and release.