I think the issue with "patterns" or "theories" when it comes to Smash is that plenty of it is guided by confirmation bias, and in some cases ignoring aspects of how characters are chosen, or the objective examples that can argue precedent and other points.
For example, the point that I've made about protagonist and/or mascot first. It's suggested because, objectively speaking, every single series or entry in an already repped series has taken the main character or "mascot" of the series/game they come from. That's 30ish data points for that example. If it were science, it would be 30 trials for a given hypothesis, all with the same outcomes. In that sense, it's pretty damning. However, this isn't really "science". That said, even if we do get someone like Max Brass or Min Min it's an exception until it happens a few more times basically.
Even then, it could be a 4D chess move if Min-Min is to become the new mascot of ARMS and be monkey's paw of still getting the "mascot".
Another example of an argument with points to refer to is the DLC choices themselves. While I think it's notable to look at how the first pick is made to start off a pass, there are other factors at play as well. Any first party newcomer that has been a part of a DLC pack has been promotional in nature. They've effectively been tied to an upcoming game, or some sort of promotion. With Corrin and Byleth, they came from the newest Fire Emblem. ARMS is a bit different because the game itself is a few years old now. Despite that, it was tied to a demo of the game and, in my humble opinion, was used to strike up conversation about ARMS and bring attention to it and breathe a sort of new life.
Data points from DLC seem to suggest mainly protagonists/mascots as well. There's a reason why folks believe these things. It's not infallible, but I don't think it's something that should be inherently "run away with" because it could happen one time. Banjo is a western-origin 3rd party, but we still haven't gotten another one. I'm not sure that it won't be broken, but with 5 characters left after ARMS, I can't imagine it would result in a huge blowout of western 3rd parties.
Just something to chew on. Not all "patterns" are created equal, but that also doesn't mean that patterns should be discarded and not given the time of day.