I think you are right the fact that some of the biggest hyped characters haven't been hinted at in a while (Ridley, K Rool, and Mewtwo) helps them in my opinion. I mean with K Rool if he is not in the game as something I would think Sakurai would say.
Well Mewtwo aside (who is universally wanted due to his popularity as a veteran who wasn't as bad as people thought and is fantastic [as top tier] in Project M 3.0/3.02), the other 2 are literally the only top potential newcomers (Chorus Men/Marshall and Shulk aside) left that could:
1. Be so unique it is stupid.
2. Have enough moves to fill out all the custom moves with "wow moves"
3. Could be heavyweights.
Now that's not saying there isn't room for characters like Dixie Kong, Cranky Kong, Krystal, Dark Pit, or Isaac, but that honestly, those 2 have priority.
On top of that, nothing makes heroes like bad guys.
The bad guys are half the story.
The "bad guys" are the reason heroes have to rise up in the first place. They're the reason people want the heroes.
On top of that "bad guys" have different play styles. This doesn't stop at straight up "take over the world and kill lots of people" villains like K. Rool, Ganondorf, and Ridley, or other "traditional" bad guys like Bowser and Mewtwo.
No, this includes anti-heroes and selfish (evil) characters like King Dedede, Meta Knight, Wolf, and Wario.
Heck, even Donkey Kong has a bit of this feel, although it's gone away with the more villain-esk characters introduced in Smash.
If you pay real close attention, all of them have an "aggressive" almost angry style in fighting.
They are generally more vicious, more focused, and have a kind of approach that doesn't suite our playable heroes.
After all, they are villains, so they do play like them.
That brings me to my last point:
4. Villains are different that heroes in not just allegiances but in how they fight and approach a fight.
And what backs up that point=???
K. Rool and Ridley are the only 2 notable villains that stand a reasonable chance at being playable.
The only other character left is Bowser Jr., a middle of the road boss who would inevitably be a clone.
That doesn't inspire the same raw, vicious, and just straight up mind melting uniqueness that K. Rool and Ridley offer in 2 VERYYY different ways.
In fact, no other characters really do offer that.