It has to be Zelda BotW 2. That would definitely put it over the edge. However, my gut feeling says there will also be a Mario project set for the holidays.
I think a new game in the 3D Land/World series. I'd prefer Odyssey 2 but a lot of people will enjoy a new World game. As long as they don't just release 3D World DX.
We should be due a new Mario game either this or next year though.
LTD: PS4 - 125m, Switch - 110m, XBO - 51m
2020: PS4 - 9m, Switch - 22.5m, XBO - 2.5m, PS5 - 4.5m, XBX - 2.8m