Assuming that Incineroar and Ken are the final two characters, the main reason that characters like Isaac, Bandana Waddle Dee, Shadow, Dixie, Elma, Skull Kid, and the like didn’t make the roster was probably just time constraints combined with Ultimate’s “bring everything back” design philosophy. That’s it, really; it has nothing to do with the fact that these characters are “undeserving,” or anything of the sort. If Ultimate was a game more focused around new content rather than bringing back all the old content, then we probably would have seen more of these characters in the base roster. However, I think it was a worthy tradeoff because now the DLC can entirely focus on characters such as these, especially since there won’t be another Smash game on the horizon to replace this one like Ultimate did to Smash 4.
The only three characters I feel have a good chance to not make DLC if they don’t arrive in the base roster are Elma, Decidueye, and Shadow. In Elma’s case, Rex will probably be prioritized over her for DLC, though that’s not as much of a probelm if Sakurai is willing to give Xenoblade two more reps, (which I think he should.) Decidueye probably won’t have to compete with Incineroar for DLC, but Sakurai might not be willing to include two fully evolved starters from one gen, and people might get grumpy of that happens, though I certainly wouldn’t complain. Finally, if Shadow doesn’t make base roster, the best we can hope for is that his Assist Trophy truly was replaced by Knuckles so that he has a better shot of being DLC. His fate may also depend on whether or not DLC echoes will be a thing; if Sakurai only wants to make fully-fledged newcomers as DLC, then Shadow’s chances diminish more. Pretty much every other commonly speculated character has just as much of a shot making DLC as they did the base roster in my eyes, if not higher such as in the case of Bandana Waddle Dee and Banjo, (which is a separate topic of discussion.)