They couldn't possibly add more moons to the game. I mean MOST people who completed the game has 999 moons. The only thing I see that is possible if they repeat something like they did in Super Mario Galaxy where if you collect all 120 power stars, you get to play the game again as Luigi. I kinda see that being impossible since they already have a Luigi's costume set in the game for Mario. Super Mario Odyssey is that game where its fine without a DLC. If anything, they might do what they did in Zelda BOTW and create a Hard Mode. If they decide to progress the game, they might just create Super Mario Odyssey 2.