If there is going to be another reveal (or maybe 2) for next E3 alongside Arms character, I think that it should be Sora from Kingdom Hearts (indenpently of his chances because of Disney).
He is a really huge worldwide and a long re-quested character since Smash 4 days (and probably the real winner of 2015 ballot without taking account veterans). Nintendo should give us a reason to spent another $30 in more Smash DLC (that is a lot for a single game) and I think that Sora is the answer. He is popular worldwide and his last game got released last year. Plus, it would tie in with the announcement of some Kingdom Hearts games coming to Switch this year and 2021 like 1.5 + 2.5 + 2.8 HD collections or even maybe KH3.
About the topic of Nintendo’s lineup for the rest of Switch life (2020-2022 min) I think that we will see:
- a lots of new entries from “Touch Generations” games like Nintendogs, Tomadochi Life, Style Saavy, Wario Ware , Rhythm Heaven, Art Academy, Jam with the Band, Big Brain Academy and maybe even the revival of some obscure IPs like Hotel Dusk, Another Code, Endless Ocean or Elite Beat Agents (let me just dream). Games oriented for “expanded audience). We can also expect some new IPs, similar to Ring Fit Adventure
- Some sequels of 2017 games like BOTW, SMO, ARMS and maybe ... Pokken?
- New spin-offs entries from Mario and Pokemon like Paper Mario, Mario Golf, Detective Pikachu 2 and a few more. (Captain Toad 2?)
- possible new entries of series that still haven’t got a new game on Switch: Pikimin 4, Metroid (2D), Bayonetta 3, Star Fox and Donkey Kong 3D (the last two maybe for Switch sucesor)
- Lots of remakes/ports/remasters of old nintendo games; SMW3D, Pikmin 3, Mario 3D Collection, Zelda WW/TP/SS HD, Fire Emblem, Metroid Prime Trilogy, Xenoblade X, Fatal Frame 2/4/5, Pokemon Gen 4 and I can name you more that would be really great to have a remake/reboot/reimagining (whatever word you like to use) like Punch Out (Wii), Sin & Punishment (N64?), Last Story, Pandora’s Tower, Baten Kaitos, etc.
- Some minor IPs (e-shop only)like Pushmo or Dillon. Here I count collaborations with indie developers like Cadence of Hyrule.
- More big third party publishers collaborations like NMH3, Bravely Default 2, SMT5, maybe Monster Hunter NeXt, maybe another Koei-Tecmo game (FEW2 or HW2?).
- More ports from mutiplattaform games that have been released on PS4/XOne.