Honestly, I think anyone far too violent like Doom Guy or Anyone from MK probably won't be looked at for Smash and that hurts coming from someone that wants Scorpion or Kitana in the game
I don't think the game being violent will stop a character from getting into the roster, but not being popular enough (or even accessible with MK, to an extent) could seriously limit their chances. Scorpion is my favorite video game character of all time, so let that offer whatever ethos it brings.
As for theories, I've concluded that only three things are reasonable when speculating:
1) Insider Information - This one has probably the least sturdy foundation. There are very few people that actually have inside information, and when I say that I mean those individuals who have proven themselves through a mostly successful track record. And even then, information can change since they last heard something. So basically just Vergeben, and even that's iffy. Hahaha. ( I know there are few others, I'm just being a troll.)
2) Theories built around money - These theories generally have the most gray area because money comes from two places: pleasing fans and pleasing partners. It is thereby reasonable to assume that Nintendo would only add the characters that benefit it
the most, but what will do that is best is the main question. Whether it's PR, partner relationships, exposure, or just sales, there has to be a reason for doing it.
3) Theories built around data or known facts - These are the theories with the most concrete foundations, that is as long as the information is verified correct. These are theories based around data contained within the actual game, SSBU marketing, official statements from Nintendo, updates to the website, or anything else directly connected to the game in an undeniable way. Box Theory and the SNK leak come to mind. Also, datamining brings with it some interesting speculation.
Any other "theories" based off of arbitrary things, such as belts or colors or genders of characters, should be abandoned as silly superstitions.