I don't really need DLC for Odyssey. I'd rather have something like Galaxy 2 which took the base game and turned it to 11. I know that some people felt it was less magical, but it's still my favourite Mario game period.
Galaxy 2 was magical to me as well and is tied with Odyssey as my favorite 3D Mario game for different reasons. However, I would prefer two substantial DLC packs for $20. It is incredibly unlikely at this point but it'd be cool to me. Have a new DLC pack with 3 new kingdoms, 120 new power moons and playable peach. There is a storyline with Peach that takes you across 8 of the kingdoms. The 120 power moons are spread throughout new kingdoms and the existing kingdoms that the story takes you to. Some new areas may be unlocked in the old kingdoms where the new dlc power moons are located. DLC pack contains lots of different capturable enemies and creative gameplay choices. Isle Delfino is one of the DLC Kingdoms. DLC pack 2 has 3 new kingdoms, 120 new power moons and playable Luigi. Same deal as the first pack along with a storyline across 8 kingdoms with new power moons spread throughout the journey.
They could then in 2020 release Super Mario 3D Galaxy. A followup to 3D World and Galaxy incorporating many elements from both. Game features the original 4 playable characters along with a new character unlocked after each of the 8 worlds. Rosalina, Yoshi, DK, Wario, Toadette, Mayor Pauline, Waluigi and Daisy.
I think its incredibly unlikely. I think they will likely go with Odyssey 2. However, with their acknowledgment of two different kinds of 3D Mario titles. I wonder if they will try to appeal to both audiences. I feel after Odyssey players would be more welcoming to the more linear 3D Mario style of game. I also think 3D World would've likely been a massive seller on the Wii and that the Wii U cut it short as well as backlash from hardcore fans that were craving a meaty, high quality adventure game. The last chance for them to announce Odyssey DLC is their next direct but I'm not holding my breath.