Unfortunately any other Metroid character than Ridley would be scrapping the bottom of the barrel, it's a statement of how poor the selection of Metroid characters is that Sylux is the one who's being touted as the next best thing. I don't disagree that Sylux would be the next in line, but consider that as of this moment he's not even a big character in the Metroid series, then imagine how he'd look in a Nintendo all-star fighting game. Obviously he is going to have a major appearance in Prime 4, but it'll take more than that if he is to claw himself out of the bottom of the barrel, at that point even someone like Dark Samus would still be a bigger character who's only disadvantage would be recency. Sylux and most other Metroid candidates also fall head first into the personality issue, not in that they're of a generic species, but in that there's nothing overly unique about them, every single Hunter from Prime Hunters is just your standard cold killing machine who move and attack the same, and Dark Samus is also only slightly better than that. The only ones who show a glimmer of personality are the Prime 3 Hunters and some of the Federation troops from Other M, but they're neither relevant, important or would make for interesting Smash characters.
The problem has always lied in that Metroid only has two major reoccuring characters: Samus and Ridley. It's been such a big issue in regards to Smash that we even have one of those on the roster twice, but it's really just the nature of the series, it's as far from a character-centric series as you can get, and so it just doesn't translate well into Smash. My wish as a Metroid fan is that if they don't intend on adding Ridley, then don't add any other Metroid characters until a viable option eventually comes out of the series, otherwise you end up representing the franchise poorly with characters who have left little to no impact on it, plus those characters would only really be added because of the series they originate from and not their own qualities.
Other than that I do agree with your post, I actually think that as far as Nintendo characters go we can get pretty close to Smash 4's newcomer cast in terms of star power, a lot of people are overstating how widely known or popular some of those characters were at the time of the game's release. We also have to consider that Smash can single handedly make some Nintendo characters into all-stars by itself.