I think it might even go without saying we’re getting a second wave of DLC. Just compare the DLC cycle of Ultimate to Smash 4’s. Smash 4 already had a blooming meta game before the release of the final DLC characters, Corrin, Cloud and Bayonetta.
Right now, we know just about all the characters already except for one. The Switch is still rather young, as well as Ultimate, and the game will probably be kept relevant as long as the Switch- or they will at least attempt to.
What I kind of expect is them adding new DLC characters of great upcoming first party Nintendo titles. A Fire Emblem Three Houses character is quite expected now for example, but I also expect something out of the new Zelda (sequel to BotW not Links Awakening), Metroid Prime 4, Pokémon Sword and Shield, and hopefully a Donkey Kong game which is yet unannounced.
I also expect new Echoes, new stages, game modes of previous Smash titles and whatever else was absent from Ultimate. The next Smash title will likely be Ultimate’s Definite Edition with some newcomers who have been highly requested but denied this time around, for example Isaac, Skull Kid, Elma / Rex, Captain Toad, Monster Hunter... name a few.