Hmm. DLC as it was framed in the Direct makes talking about who they could be tricky. I've resigned myself to the disposition that ultimately it doesn't matter to me, since I'm a big fan of a lot of the characters new and old, but I did notice something highly unusual.
The Assist Trophy line-up is exactly 59 characters, as stated both in the Direct and on the official website. It's a very odd number to end development on. And looking on the official site, the list includes vets from both games like Waluigi and Samurai Goroh, returning Smash 4 vets like Skull Kid, and even bringing back old favorites like Isaac from Brawl (not in the form most people wanted, but still an old favorite). I ultimately noticed two things:
1. Custom Robo, despite being referenced in Melee and Brawl with trophies and Ray MkIII as a Brawl Assist Trophy, was completely gone in Smash 4. However, in Ultimate, Custom Robo is back in force with a full compliment of spirits and even a Ray MkIII costume for Mii Gunner, but the Assist Trophy is still absent. I take this to mean an accurate representation in AT form would be unfun because Ray was kinda bad since he was so small and his attacks so easily dodged, not necessarily that he's going to be one of the DLC characters. That'd be awesome for me, but I know the Custom Robo series has limited appeal. It's clear to me Custom Robo is referenced at all because trophies were cut.
2. Saki, one of the few Assist Trophies that was in both Brawl and Smash 4, is not in Ultimate. Now I don't know anything about Sin and Punishment, but I have to assume it's more popular than Custom Robo given it's had a VC re-release and a Wiiware title that came out later than CR Arena. And yet, Saki and co. are nowhere to be seen. I think this is deliberate; in fact, I think Saki was AT #60 and was cut last minute to be promoted to a DLC character, with a free content update featuring S&P spirits and references releasing alongside it. This is all the same circumstantial evidence that seemed to suggest Shadow had been promoted, I'm aware, but Sakurai has paid lip service to Saki being a viable choice as a fighter before.