This is certainly a plausible theory, especially with the dismembered torso for moviegoers to examine. The presence of the red-clad Imperial guards also lends credence to the notion of Vader being the medical patient, since their appearance typically coincides with someone of high importance to the Empire. As for who is visiting the bacta tank and why (allegedly) Vader requires the treatment remains to be seen until Rogue One is released; though, SWNN has some ideas about the latter of those two that are worth reading – including regular therapy sessions and wounds suffered during a battle.
As stated above, the bacta tank (if that’s what it really is) imagery has been conspicuously absent from all of the trailers and TV spots that have come out since the summer, causing some to theorize that the scene may have been cut following the extensive reshoots Rogue One underwent. However, Lucasfilm probably included it in the first preview for a reason and may have chosen not to elaborate on it further to preserve any spoilers. All will be revealed soon, and fans will be keeping a close eye out for this sequence.