I doubt the Pokémon Trainer was that hard to implement. Afterall, the individual characters are quite well fitting in. Outside of maybe Ivysaur, who's still leagues easier to fit in than Ridley still. Doubt Rosalina is as much of an hassle either, her Lumas could be tricky but it's not yet on Ridley level either I imagine.
Hardest thing would be to balance Ridley. And that's the whole issue here. Would be cool to see Ridley. However, with the whole stage hazard thing being likely and all, am not really expecting a lot.
It's not as much in the visual design phase I'm thinking in regards to Pokémon Trainer, but more so in the moveset, coding and animation phase. Each Pokémon had to serve a purpose that the others couldn't, they had to fill individual roles, and at the same time they also had to work and flow well together in order to not make the whole character feel like a clumsy mess. Then there's also things like loading the PSA and model of the individual Pokémons mid-battle, which the Wii does have some lagging problems with. It's a much bigger project than it initially comes off as. All while they also had to try and balance each Pokémon to be just about as strong as each other. As we can see with the final product, it's far from flawless, but I say it's still an extremely well done job considering how hard a character like that is to make. Oh, and supposedly Game Freak is also hard to work out rights with as Sakurai has commented, so having to work out rights for 4 characters just to get one character working must also have been quite hard.
Rosalina probably isn't on Ridley level, at least during the design phase, but my comment on her was also mostly in reference to having to have two characters do different things on one input of a button, mostly in regards to movement, while also making it feel fluid. It's probably not like Nana who's just an AI that mimicks Popo, I imagine the Luma has a unique file that is somehow linked to Rosalina's and acts like a second character being played.
I'm not sure what you mean with balance though. As in he'd be handicapped by being a big and slow character? Well, seeing how poorly Bowser has done competitively in Melee and Brawl, then yes, it could be a problem. I imagine making him slow on ground, but quick in the air could potentially fix that, the problem is mostly when a character is slow all across the board while also being a big target. Ridley's difficulties mostly resides in the design phase, but if they can make it past that then he shouldn't be too much of a hassle from there on out. I do see your concerns though, so don't get me wrong on that, but I do believe Ridley is possible judging from what Sakurai has made work in the past.
Ridley is obviously the kind of character I love to discuss, but there really isn't much to do than to wait and see what happens.
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