Not sure if this has been said before, but here goes nothing...
What if Sakurai was working on Ridley since Melee?
and before you say he wasn't even consdiered as playable in Brawl, I have a reason (manly because he wasn't, but I'll get to that)
Again could just be me seeing things, but as I stated, the Models for Samus and Ridley in the Melee Intro, and Samus with a Mushroom in the SSE fight seem to have more or less the same ratio. So what if Ridley was slowly being tweaked over the games since Melee? Since in Melee he only appears in a GCI Intro and in Brawl he's a Boss, but wouldn't that have given Sakurai and the rest of the team plenty of time to work out how to make all of his portions the right size, and the only thing that seems to have changed between his Melee and Brawl visual appearance is the detail, even his wings seem more or less the same ratio (to the rest of his body).
Then we have the picture of Samus in the SSE fight, that proves Ridley scaled down with Samus to her normal height it would work, by scaling Samus up with the mushroom. But for the fight with Ridley in SSE I remember @
Phaazoid (I think) compared it to Dyna Blade, and since I haven't fought against dyna blade in a kirby games, I went and watch a video (which I now can't unsee, thanks). Granted not all the moves would translate over to a playable version of the Ridley, but things like the tail jab and ground pound moves could, plus they could base the scale off Melee and Brawl Ridley that I just went over.
So basically the main assets Sakurai would need to make Ridley are already there, if he was ever intended to be a playable character or not. Maybe they were slowly working on Ridley but they didn't have the time, which could explain why he's in the Melee Intro, same with Brawl but they just used his model (and some possible intended moves) to make a boss instead. Though the main issue I see if making a move set that would work in Smash, along with a movement system that fits Ridley him, like not making him broke or move awkwardly, which I think hovering would do, his tails closer to the ground than I initially thought, so hovering is a possible choice. And he has to have moves that fit is character, without being too similar to other character move from Smash.
Another thing to point out which never made much sense to begin with, the people who claim that Samus should be the only Metroid rep because Metroid games are meant to be about isolation, why is Ridley with her in the Melee Intro?