Nintendo Smash RPG open world game with staple characters.
Hardcore photo realistic gritty Pokemon RPG open world.
Paper Mario (if they return to TTYD formula)
Any of these will be 10 mil plus series if done correctly.
Hmmm... not sure if serious :P
Iirc the best selling Prime game is under 3m, so 4 would have to actually smash the previous highest seller out of the park. Not sure if it has broad enough mainstream appeal to do so but I guess we could get a "Breath of the wild scenario".
If it's done correctly and marketed well I think it could. GameCube bombed so the first two can't be blamed for bad sales. 3 was controversial if I remember and not well received, plus Wii more only limited it's sales. If 4 has optional joycons conytrolls, not required, is a visual masterpiece, and is marketed well then it could be 10 mil. Also if it doesn't release in the last year of Switch life, maybe even last two. It needs time to grow before a new console comes out unless it is a launch title itself.