Hopefully the Ultra Sun and Ultra Moon announcement can be a way for GAF to keep their expectations in check for next week. Nintendo will have a solid E3, but it's not going to be a show filled with your deepest first party fantasies. Expect a small number of new announcements and enjoy the extensive coverage of games we already know about. Which is fine.
A surprise or two beyond what we know would be most welcome, especially since there are some big question marks that seem due for answers (i.e. the rumored SSB, a new Animal Crossing, the rumored Pikmin, whatever Retro's been making, etc).
But yeah, if nothing else... Super Mario Odyssey looks strong enough that, if this were 2016, it could've carried its own show much like Zelda did. So we're getting a blowout on that, PLUS finally seeing Mario + Rabbids in action, and more than 10 seconds of Fire Emblem Warriors, and likely more on Xenoblade Chronicles 2, which has been repeatedly confirmed for 2017 in all regions.
I also think we'll see new stuff on Splatoon 2. It's their next big release for Switch after ARMS. They showed Hero Mode last time, which makes me think they must have some exciting stuff to show for E3.
Beyond those, after the Spotlight, I'm looking forward to the three invitational tournaments (Pokken DX, Splatoon 2 and ARMS), and more on Pokken DX itself during the Treehouse. Treehouse is confirmed to run all three days and will likely showcase some neat 3DS titles, too, including Ultra Sun and Ultra Moon, and quite possibly the previously announced Kirby multiplayer action title (teased in the April Direct), and maybe the Mario & Luigi: Superstar Saga DX that was found in eShop listings.
The best part is, regardless of what happens, ARMS comes out at the end of the week, literally the day after E3. Also, we're apparently getting a free Pokken DX demo during E3.