This is assuming it is a main series game, which while it hasn't been confirmed nor denied (they said nothing on the matter), it's pretty unlikely that it'll take place within the main games continuity at all.
That being said, though, to answer the thread's question, I would probably place it in the Downfall Timeline, simply because of the art style. TWW-PH-ST, same art style. ALttP-ALBW, same artstyle but updated. OoT-MM, same artstyle. ALBW-TFH, same artstyle. Seems to be a trend. We don't know anything about the game storywise, so that's all we have to go on. Because of this, I think that's the safest place to put it.