I love you all to death (Just so I can smash bros with you) (SSBWiiU/3DS)

[Terminal Devastation]

Ha. Wii fitness trainer. SIlly, and helps the gender ratio. That's thinking outside the box.

I liked Tonberry2k's little predictions, so I'd like to try my hand at some of the ones I don't quite agree with him on.

Kirby has all of the characters it needs. It theoretically has room for one more, and if so, it'll be Bandana Waddle Dee. Can we all agree on that?

I agree with this for the most part. Like you, I find Kirby has all the characters it really needs. There's others I'd like to see as well, but I also think if Kirby gets any it will only be one. Where you and I differ is who would be the next Kirby if any.
Bandana Waddle Dee has many of the same disadvantages you mentioned for toad. Slight lack of a moveset (and nothing suitible he can borrow from Kirby either) and mostly being represented in Dedede's moveset are decent strikes against him showing. I'm leaning more towards Magolar actually. He may break the pattern, but Kirby doesn't have a suitable love interest, and Magolar has more than enough moves.

As a side note: I'd really like to see an Adeliene assist trophy, if assist trophies return.

Metroid is all about Samus. With two versions of her, it's difficult to imagine we'd need anyone else. But still, people have been clamoring for Ridley for years.

Ridley: The space dragon. He's cool, he's terrifying, he's Samus's biggest rival. Size isn't really relevant, as Olimar and Bowser are both in with adjusted sizes. So where's the problem?

Positives: Metroid's most recognizable face after its protagonist. He's the Xenomorph to Samus's Ripley.
Negatives: But when you get down to it, what does Ridley offer that Charizard doesn't already? He can fly and shoot fire. They both can. If there's some major reshuffling, I can see Ridley making the cut, but as-is, this story is all about Samus and everything else isn't as important.

Hunters: A bunch of space bounty hunters who appeared in some Metroid games.

Positives: They have names, I guess. And viable movesets.
Negatives: They have zero personality and aside from some cosmetics, each of them plays exactly like all the rest. Nothing sets them apart or makes them interesting.

Verdict: Two Samuses seems like plenty from this franchise.

There's two reasons Metroid needs another character: First, it's one of the bigger IP nintendo has. Regardless of the character focus, people want more characters from it, just to show it's getting the same amount of love. Really, if metroid had some more suitable reoccuring characters, they'd be a sure in.
Second: I'm pretty certain there's a lot of people who don't find ZSS to count as a second metroid character, myself included. Really, ZSS is just an expansion on Samus, the same way Shiek is one for Zelda, and Charizard/Ivysaur/squirtle are all the same character "pokemon trainer". Yes you CAN play them as a separate character, but really you have access to both as a single character.

With that said: You've already covered all pros and cons of ridley as a playable, and he was well served as a subspace boss, so it would be nice to see him return as such if not playable

I'd like to highlight some of the stand outs from the rest of the cast:

Rundas: If he had a longer run, just from what little we know of the guy, I could have seen him turning into Samus's rival-sidekick so to speak. That gives him a bit of a boost compared to the others. Also could serve as a peace offering saying "We'll be more like Prime next Metroid game"
Pros: From the last good Metroid game, enough moves for a full moveset, and a unique one at that.
Cons: Older game, really not that important to it either.

Anthony/Adam/GF soldier: I can already hear the groans, but this is a real possibility. If Other M was actually a GOOD game, Anthony/Adam would have been a sure in. Ones the closest to a sidekick Samus has ever had, the other is plot significant (I think Anthony has more chance to show up to fight in the future though, so I think he'd be the better fit). There's two ways I can see the moveset coming together as well. The first is only an option if Snake isn't returning. All they'd have to do is reskin snake into a metroid flavor and maybe adjust a few moves (and the taunts), and they'd be done. The second would be to take the few know abilities he has in game, add stuff the GF troopers had in Prime 3, and fill out the rest with gear Samus has but isn't using in brawl (Like what Zelda did).
Like Samus's Shinespark, or Diffusion Missile. (Adam/Anthony would definitely have Ice beam though).
Pros: Actually have significance to the series, green color could poach Halo fans, decent material to work with for move set.
Con: From a really bad Metroid game.

Personally: Hoping Rundas or Ridley.