They can still mess with the proportions as needed. Ridley's design is never consistent between games (save for Meta Ridley, who gets a Phazon related makeover in 3 anyway) so most design changes wouldn't stick out too badly. I'm personally hoping they go with a composite design. (say, zero mission colors, blank eyes [pupils are for chumps] and close-to-brawl proportions)
Hmm yeah that can work actually. I haven't played all of the Metroid games but I am familiar with some of his designs. Zero Mission seemed like one of the best.