FancySmash
Smash Lord
Oh, yeah, I get it now. Relevance for the series (first party anyways) is almost a must have for some reason. It's as you said, Golden Sun hasn't gotten anything (aside from Isaac's assist trophy back, a hollow victory) almost certainly because of that. I think Advance Wars suffers a similar fate, that and it's sword series brother kinda has it's foot in the door.FancySmash It seems more like the series needs to be relevant, not the character alone. DKC never stopped being relevant as is. Meanwhile, Golden Sun has yet to get a playable character, and it definitely wasn't relevant during Ultimate. It was lucky enough to get a lot of content, though. If Golden Sun relevant, even if Isaac wasn't there, he'd have a way better chances of getting in.
Take a look at our Castlevania characters for a good example of that; Castlevania was kept relevant for a while now(the tv show and Pachinko machines obviously don't tie to Simon or Richter, but the series is more important alone). If both didn't exist, which I still believe wouldn't really affect Simon and Richter getting in, the relevancy argument being dead would actually hold a lot more water. Right now, it really doesn't, especially for 3rd parties. So far all were currently relevant when added to Smash.
Which is ironic, since it was never mentioned once as a criteria towards them(just 1st parties. And we definitely see that relevancy has affected 1st parties before. Wolf was unfortunate timing because no new games were coming out soon. Falco probably had enough popularity and veteran status(via 2 games) to stay in, coupled with being a semi-clone. Lucas is an odd one, but no new Mother games, or even re-releases, were coming out soon. Ness obviously being cut would be silly since he was there first at that point. One thing was Melee, where Sakurai was more willing to cut characters, even those who were there before similar ones, but Brawl and 4 were a bit different. Other than semi-replacements, like Toon Link for Young Link, cuts were done a lot more carefully. Mewtwo is another oddball, as it's not clear why he wasn't in Brawl or why he was lower priority than most. He was probably harder to program at that point, after you got the major ones in. Jigglypuff obviously was an easier one to do so. So lower priority. Could also be Gamefreak or something).
As for Mewtwo, well, we know he had data leftover in Brawl, enough to apparently complete him for Project M. Brawl's development was likely the most hectic of the Smash games. It was delayed at one point, and Sonic's late inclusion didn't help much either.