Here are some things we might see in 2020.
1) The usual suspects - games that regularly come out on handheld or home consoles.
Wario game (either platformer or smooth moves)
Pokemon Mystery Dungeon (or other Pokemon spinoff)
Proper 2D Mario game (non-port or non-Maker)
Proper Donkey Kong Country (non-port)
Metroid Prime 4
various Mario Sports titles
2) Revive old franchises
Wii Sports (or something like it)
Lots of others. Too many to count.
3) Ports & Remakes
There are still a few Wii U games left to port. On top of that we got Link's Awakening this year and Xenoblade Chronicles is on the way. Nintendo will likely have at least one other port in 2020 either from the Wii U or an older console.
4) 3rd Party games using Nintendo IP
The Switch already has Mario + Rabbids, Hyrule Warriors, Fire Emblem Warriors, Starlink (w/ Starfox content), and Cadence of Hyrule. They are showing that they are actively trying to get 3rd parties to make exclusives for Switch using Nintendo IP. I doubt this is going to stop.
Pokemon, Mario Party and Kirby sequels are definitely going to come eventually (though not necessarily in 2020), and we already know about BotW. Mario Odyssey, Mario Kart, and Splatoon are all games that may or may not get another game on the Switch. Also, while technically a Square Enix game, I know the Octopath Traveler team is working on a sequel or some game like it.
6) New IP
So far we've gotten ARMS, Snipperclips, 1-2 Switch, LABO, and Ring Fit Adventure. (I may have forgotten a game.) Nintendo is putting out at least one new IP a year. I would not be surprised to see this rate increase either considering how successful the Switch is. Nintendo can afford to be more experimental at this point.
In short, Nintendo has a lot of games that they can still release including several heavy hitters. They are in no shortage of potential content. This is not even counting the release of 3rd party content, which has been accelerating each year. And the Switch is successful enough that Nintendo can take some risks starting new IP or resurrecting old IP. In fact, I think it is very likely we will see at least one game from each of the categories I listed.