The only problem is Ridley's size.
The size he was in Melee's introduction could MAYBE work...but in my opinion, it's like turning a T-Rex into a raptor. Both scary and deadly, but they're just not the same, and they really wouldn't fight the same way.
Ridley's supposed to be quite big, much bigger than Bowser's ever been (Giga excluded of course hehe), so it really would be interesting to see how they'd make a playable Ridley work. I'm honestly only expecting an AT, Adventure Mode, or stage appearance like with Kraid in Melee.
For a stage, it could be something like the Tallon overworld or similar location, and the stage could change places much like other Brawl stages have been doing...and in each different spot, Ridley would come and attack in different ways...much like the early fight in MP3.