I don't care much for Bowser. Jr. He's a character who had potential and I loved where they were going with his relationship with Bowser. It was genuinely cute and I'm glad they didn't go with the "THEY'RE BOTH BAAAAD SO THEY DON'T GET ALONG" route, or the "son is incredibly obedient and lets the father control him, but the father doesn't value him because evil" routes, as they are really cliché. But like most characters who get introduced in the main Mario series and don't get in the RPGs much, their potential is squandered from then on. Bowser Jr shows up in Galaxy and NSMB games, but he just felt like a generic mook with no personality. And he hardly feels like Bowser's son, he's just there. Bowser never even ACKNOWLEDGES HIS EXISTANCE in the Galaxy games, from what I recall.
And I've already made it clear that I don't think moveset potential and a large game portfolio are at all criteria for getting into Smash, as Sakurai could make good with any character moveset wise, and appearing in lots of games doesn't mean a character's appearances were anything special.
Rosalina, is the one exception to the "Mario platformer characterisation" rule. Her character was portrayed in a genuinely charming and mysterious manner from beginning to end, she had a great purpose, and she had a beautiful backstory that
dealt with the theme of tragedy IN A MARIO GAME. She may have only had a prominent story role for one game, but it left a mark on most people who experienced her story. The Sunshine ending was cute, but that's it. Rosalina's story and everything to do with her, made me feel a bunch of things, on both the happy and the sad side. And although she's appeared in only minor roles since, they at least expanded on her character in subtle ways (for example, she certainly knows how to have fun, and can be tease if she wants).
And anyone who doubted her moveset potential was blind. She took out Bowser's air fleet single-handedly, and she has control over space. There were tons of possibilities for even a solo moveset, let alone one where she summons Lumas to help her. A character doesn't have to actually have performed 20-odd signature moves to be considered viable for a Smash roster, they just need a distinct theme to work with. Remember, Captain Falcon's moveset was made for Smash, and Captain Falcon has barely engaged in actual fights outside of his Blue Falcon in any other media. In fact, most people remember him for his Smash portrayal.
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Also, this may give Snake believers some hope - it's old news, but when the Wii U launched, there was a glitch that allowed people to access a debug version of Miiverse. Only a few people encountered it, but there were some prototype communities, one being "Metal Gear Solid". There was also "Yoshi Land Wii U", and as we know, a Yoshi game for Wii U was announced in a Nintendo Direct shortly after.
http://www.destructoid.com/wii-u-user-accidentally-hacks-miiverse-for-a-minute-238932.phtml