Im not entirely sure about Sylux, but i do know Rhythm Girl and Porky are effectly big main characters in their respective series. Not only this, but Gigyas is even missing from the spirits list, whom id assume would be a theoretical Porkys final smash or associated with it.
I think people are taking the "promotional" aspect of DLC way too seriously. It never HAS to be a promotional character. Just even looking at the base game, Ultimate added Ridley and K. Rool, and while they have popular fanbases, they have hardly been relevant in recent Nintendo history. Same goes for Simon and Richter. Castlevania is popular, but lets not forget how Konami essentially dropped the series and Simon/Richter are some of the oldest Belmonts. Even Incineroar is about to get phased out nexr year when Gen 8 comes.
I can tell you from a General Audience perspective, MANY people had no idea who Ridley, K. Rool, and Simon were before Smash, yet they are new inclusions made for the game. Their designs and gameplay alone sold people on them. Its exactly how character like Ness, Roy, Pit and IC became popular over time.
All im saying is we only have Smash 4 DLC to go off of in terms of predictions, and even that is hard to base anything off of because it had 3 vets, 2 extremely iconic characters in the form of Cloud and Ryu, who were promoting nothing at the time, Corrin who was a literal "who?" to anyone outside of Japan, and Bayonetta who is not NEARLY as popular as people make her out to be (and she was promoting zilch since her game came out 2 years prior and to this day has sold only 700,000 copies with the switch version too...fyi its not alot.)
So honestly, while basing anything off of spirits is silly in the long run, its not shocking to see Nintendo picking random characters that look fun to play. Why? Because people will gobble it up anyway. I have never imagined K. Rool or Simon to be this popular after Smash. Nintendo could literally make anyone DLC and sell it extremely well.
So yea, we CAN get Sylux, Porky, Rhythm Girl due to their conspicuous absense out of 1300 nintendo or nintendo affliliated characters, especially when their own property has much more obscure characters. We CAN get a promotional rep because why not. We CAN get more iconic characters (i am personally hoping for Banjo). There really isnt a pattern. Literally anyone is up for grabs.