The problem with that is that Rain's story is tied to Daegon and Taven, so I don't see NRS developping him without including his two brothers. They could have him aid Shinnok's conquest of Edenia, like Tanya, but then don't expect him to have any impact on the storyline besides being a low level goon jobbing to the good guys.
As far as I'm concerned, I agree with Marcelo, I wouldn't want to see the post MK3 characters forgotten. Sure they sucked --for both gameplay and story purposes-- but let's be honest here, so did the original characters in those games. In fact, if the pre MK3 characters debuted in the post MK3 games, I can almost guarantee that people would have found them forgettable, Sub Zero's invovment in the post MK3 plot was laughable, they took away Kabal's most noticable feature, his speed and gave nothing to replace that. So yeah, some could really use a second chance away from these crappy games.