Ugh, I dunno, man... The thing that I've realized about Smash Bros.... is that it's just a glorified popularity contest. I don't think it'd be even slightly realistic for Impa (a character who may be important to her respective series but barely iconic nor popular to the general audience) to take the place of Sheik. If anything (yeah, I'll probably get flamed for this, but whatever), Tingle is the most realistic character to be added to the Zelda series as a newcomer. Why? Because the guy's super popular, well-known, and will truly get people hyped to see playable. That, from what I believe, is what should be looked into when it comes to potential newcomers. NOT simply how "important" or "relevant" they are.
Look at Luigi. Was this dude even remotely important to the Super Mario series when he was made playable in Smash Bros. 64? Nope. He was totally just a palette-swap for Mario in 2-player mode. Did he even need to exist? NOPE. They could've just slapped in another Mario and added a "2nd player" tag to his name. Would've been the same game. Luigi had never truly impacted the Super Mario games at the time to be labeled an important character to the series, but guess why he was added? Popularity, man. It (from what I'm seeing) is the most prevalent factor in why a character ends up being playable. Which is why it really saddens me to see people shove aside characters like Tingle and yes, even freaking Waluigi, just for not being important to their respective series, nor making much if not at all of an impact on it.
The whole concept of Super Smash Bros. is just this; iconic characters beating the living **** outta each other. Emphasis on iconic. Characters that should be considered strong contenders are the ones that people will go nuts over if made playable. Rosalina suddenly being listed as the most likely Super Mario newcomer (if there even is one...lol) simply because she's been added in SM3DW as the secret character isn't really a valid statement. Yeah, she's playable in a mainline Mario game. But honestly, she's a pretty damn obscure character at the moment. She is added as a character that can only be seen and used AFTER you beat the entirety of the game, and is barely even important in the Galaxy series (she's no more important that Lubba in Galaxy 2, and only appears in Galaxy 2 at the last goddamn second). This is not to say that Rosalina is, by any means, unpopular, but popular and recognizable enough for Smash? Maybe we need to rethink that one. To be honest, I also felt that Rosalina being a newcomer was inevitable, but after realizing what Super Smash Bros. is truly about... not so much.
If Waluigi was considered popular enough in the Brawl era to receive stickers, a trophy, a color swap for Luigi, a remixed song, and Assist Trophy status (literally the closest thing to being playable), what makes him so super-duper undeserving of being playable in a future Smash title? He hasn't appeared outside of a spin-off? Well the dude's somehow become pretty damn popular regardless of that. And that alone (in my opinion) should be enough to be given a chance as newcomer. He's recognizable, he'll make the general audience more excited if made playable than Rosalina would (once again, not labeling Rosalina as "unpopular", just not as iconic), and would flat-out make more sense (for the lack of a better way to say it) to be playable in Smash Bros. 4. This is also why I don't see Bowser Jr. as a likely candidate for Smash. If anything, the most "deserving" newcomer would've been Toad. But since he's retained his status as a "meat-shield" and is no longer seen as a realistic newcomer... I have no idea. I guess Paper Mario (if made a Super Mario rep instead of a Paper Mario rep) would have to be the runner up at this point, with Waluigi closely behind.
tl;dr: importance is not what we should be looking at when it comes to newcomers... popularity and how recognizable they are, is.
(Holy ****e, when've I ever typed this much in my life)