Nintendo has toned down on the whole E3 thing since they started doing Nintendo Directs so this one won't be any different, but that doesn't mean they won't have things to show off. This is a quick list of what I expect Nintendo to bring to the show, so in no particular order, lets begin.
I think the big four will be:
Super Mario Odyssey
Zelda Breath of the Wild DLC
All of which will get a new trailer and be playable on the show floor. The trailers will reveal more about the games and new stages.
Other notible entries will be:
Pokken Tournament DX
Pokemon Ultra Sun and Pokemon Ultra Moon
I also think they will reveal a new game:
2.5D Metroid for 3DS
3D Metroid for Switch (not a Metroid Prime 4 but people will still subtitle it as such)
The reason for a 3D Metroid on Switch would be to give the Switch a good variety of games as I don't think it really has a big shooter right now, however since I'm not big on FPS games I'll be more excited bout the 2.5D Metroid on 3DS.