Metroid... has been and currently is a flaky series. While having mostly well-recieved games, the series goes for serious gaps of time without any titles. Since Other M's very heated release, all we have seen is the announcement for the spin-off of Federation Forces. This is a series of fairly high demand for a newcomer but it's not on the best footing, as scary as that is to day.
Ridley: The Elephant in the Room. Ridley, being one of the single most requested character for Brawl and Smash 4. Neglected. Why? Sakurai's reasoning in Brawl being that he didn't really work through the idea and that "it'd likely be impossible; but if it were to happen... he'd be a bit slow." This got affirmed after the release of Smash 4. Sakurai's interview basically stating that he felt that the intimidation that comes from Ridley fits far better in a Boss form than that of a fighter. But... let's seriously talk about Ridley's obstacle(s) and what changes would benefit if not be needed.
A lot of people dumb down Ridley's issue as being "Too Big." Let's.. look at this. In Metroid NES, Ridley was the same size as Samus... in a form that HARDLY resembles the character we know and looks more like a mutant Birdo. In later 2D games, Ridley is seen being about 2 to 3 times Samus's size... but most often very "crunched up," with his attacks namely being his flame breath and deadly tail instead of full ranged movement... partially due to limitations. The best we've seen Ridley move organically is when he's at his largest in ratio to Samus; the 3D titles. Prime 1, 3, and Other M.
Yes, Ridley is large and intimidation comes with that size. But it's his feral attacking and the raw amount of damage he could dedal that left a stronger impression on fighters. Mechanically there's a matter of, in terms of Smash, him having to be a pretty lightweight character with high speed (most likely being a heavilly-aerial focused fighter and a bit of a grappler as well) but also being around the size of titans like Bowser and DK. Just conceptually, this is a bit off... but feasible. What comes as the biggest issue of trying to impliment this kind of stance.... is Ridley's design.
The Super Metroid Ridley that carries on to almost every other incarnation... is incredibly scrawny. Anorexic looking. The size helps hide some of the issue of this, in Metroid.
But in Smash, even if you just scale him...
The size in general is not that much of an issue. It's how his neck looks like two individual bones and he lacks the physique that fits the kind of power that would be "ideal" for that playstyle.
Of course, the biggest example of how it CAN work, and in part due to the modding effort of
@BaganSmashBros, is using the Other M "Roidley," which... in spite of some of the outcry against it, has more considerable bulk to him. I'm not saying turn Ridley into Bowser. Just give him a body that could fit better as a fighter.
This should show the kind of difference I'm talking about. It's not even that major in bulking up; but it's enough to make a significant difference and one that retains an intimidating figure at a fighter's size a lot better.
If we see a Roidley-esque design "stick" with Metroid fans, or Sakurai no longer at the helm for Smash 5, Ridley is a damn likely choice but short of either I will remain a detractor because his issues haven't gone away.
While some people may argue Dark Samus, Rundas, Sylux, and even occasionally Mother Brain... let's be serious. Ridley is the only Metroid character that has all 3 of the biggest things for a character in a series without a revolving door of characters.
A) Request Power
B) Series Impact
C) Reocurring Roles
Dark Samus is dead and likely not returning. Rundas and Sylux are questionable on "series impact," and one is dead after his first game and the other is only teased to be in a future 2nd title. Mother Brain... lacks the request power and certainly would be more obscure on how to impliment as a fighter.
For Metroid; it's Ridley or Bust.
Ridley, however, has his body to get more in line for smash to be seen as likely, to me.