I feel like the only true “rules” of Smash “speculation” come down to how we use language or just assume likelihood based on what we want to be likely.
1) The concept of “deserving” a spot in Smash implies that there are spots for series as Sakurai sees them “fit for Smash” or that Smash is some sort of fabled Gaming Hall of Fame that excludes games that are clearly hall of fame games, like Halo, God of War, Uncharted...true quality titles that will likely never be in Smash.
To deserve means to have earned something, and to “earn” something one has to actually engage in some sort of activity. This doesn’t really work when you realize the game itself is not sentient. The game itself did nothing, the creator(s) did something amazing.
Basically, a character can’t really be “deserving” in the traditional sense.
2) “Likelyhood” is based almost entirely on what we perceive to be true, mixed with what we want to be true. The Grinch Leak and frankly the extreme rise in “Steve is in” has proven this to me at the very least.
Likelyhood for Smash has no definition, and it would be hard to create one that didn’t anger one side of the fanbase or another. There mainly assumptions based on what we literally see in the game, but we really won’t know what is “likely” for Ultimate when it comes to characters until it’s too late to figure it out in the first place.
Is Waluigi likely for DLC? We can’t really say. It entirely depends on the design of the game itself. Does the dev team see Assist Trophies as an ironclad position once programmed? We won’t know until Ultimate is done done.
3) Not really a “rule”, but something I think needs to get buried is the “relationship with Nintendo” rule. Nintendo is going to have 3 characters who are hardly seen as Nintendo gaming mascots, and they’re 3 characters that don’t even necessarily fit the Nintendo “style”. I say “style” because i’ve been told that Snake wouldn’t come back because he didn’t “fit” with Nintendo, like Sonic, Mega Man, and Pac-Man did.
Then, each subsequent 3rd party was validated because “muh Nintendo connection”, which is dumb. Cloud, Snake, and Joker are hardly Nintendo at all. They are gaming characters that come from successful games, and bring something interesting to a crossover game that brings a ton of different ideas together.
You know who else looks “out of place” next to each other? Mario, the guy with a huge head and nose, and Ike, a normal looking human who can wield a 2 handed sword with one hand.
It’s a crossover. The characters being different is the entire purpose.
May have gotten carried away, but I don’t think we’ll have concrete “rules” until after Ultimate is done. Nobody expected Joker, and while we have an idea that Erdrick is coming and Dragon Quest is a “legacy” series, there still simply isn’t enough available to make a serious claim as to what we’re going to see to cap off Smash Ultimate.