It could be a number of things really. It appears to be a catacomb beneath Hyrule Castle, which is where Calamity Ganon first appeared in BotW. This would make a Ganondorf resurrection of some kind not too far fetched. He was a fan favorite after all, but only appears in 3 mainline games.
However if the game takes place outside of Hyrule, being pulled into an alternate reality via some new evil entity is also very possible. This would not be the first time this type of event has taken place within Hyrule afterall and given the theme of the trailer, a journey to a neighboring land does not seem likely.
As to whether or not this means a return to Termina however depends on if this is actually connected to Majora or not, and there really is not enough evidence for me to comfortably back that theory up. Future trailers will make this more clear but FOR NOW, I am going to say no to this being Majora.
No matter what the case is however, this announcement made my day. It is no secret that I and many others have suspected that a MM style sequel with reused assets on Switch would fit well with the timing of this E3. Saying I am hyped would be a massive understatement.