I'm sorry, but this "pattern" simply does not exist. What you've described is a layout of 2/1/2, 5/4/4, and 5/5/5, that's not a pattern, that's for all intent and purposes a random assortment of playable characters. If you could at all point to any concrete patterns (i.e. Mario, Pokemon, and Zelda sharing the same number of representitives throughout the Smash franchise) then I'd agree you have something, but there's no credence to what you're saying.
Sakurai has never stated that Mario has to have the largest number of playable characters or that it can't be surpassed, and there's no evidence to suggest that will remain the case. Of course, basing this on slot space is trivial when there were already more characters playable from the Zelda and Pokemon series in Brawl. And dedicated real estate? Roster spots are a digital commodity, not lakeside condos. You can cram an awful lot of respectably sized roster spots on a single screen.
Admittedly, that's true. It's hard to say what was the cause of Mewtwo not being included in Brawl. The character does have more assets than any of the other character files that were found within the confines of the disc, however, and if Sakurai truly planned on making him a boss then he would have – meaning Raquaza wouldn't have filled that role.
As for not fitting as playable material? What?
Sakurai counts transformation characters as separate characters and he has for years. He has stated that Brawl had 39 characters in it (
source), so the "finite roster slots" argument doesn't hold up. He's finally separated them, grouping them together would be silly because they're wholly original characters. It's about time he finally separated them.
As for the fact that they'll be taking up their own slots "
ON THE SCREEN," I'd like to say thank you. I honestly believed they'd show up somewhere else.
Ribbing aside, your points hold no validation. All we are is hopeful that Mewtwo returns, and given the fan outcry for such (not to mention the immense relevancy the character now has) it'd be odd if it didn't make the cut. Of course, there's no predicting Sakurai, and with E3 2014 quickly approaching – as well as the 3DS release – we'll know sooner rather than later what awaits fans.