Ridley's probably my most wanted newcomer, but I do think the likelihood of his playability is far overstated. He was never considered as a playable character for the first three games, and when asked about Ridley for Brawl, Sakurai replied, "I think that would be pretty impossible." He was given two boss spots in Brawl and there's even evidence of a scrapped Assist Trophy -- clearly that's as far as Sakurai's willing to go. He thought Zero Suit Samus made for a better second Metroid rep in Brawl.
Ridley's far from a fan favorite icon to the general public, and he's not very popular in Japan, so unless Sakurai scours American forums for roster input, I don't see what incentive he'd have to include Ridley. Brawl was designed to be Smash Bros.' last hurrah, and Sakurai stuffed in all the "deserving" holdovers: Wario, King Dedede, Pokémon Trainer, Diddy Kong, Meta Knight, and Wolf.
The only newcomers I see happening are additional third parties, retro characters, characters who didn't exist during the Brawl development period (Chrom, new Pokémon), and Palutena, who experienced a recent revival after twenty years of inactivity. I maintain that if Sakurai thought Ridley warranted a playable character spot he'd have been included in Brawl. (I feel the same holds true of characters like Toad and King K. Rool.)
Hopefully I'm wrong, though. We'll see in five months.