Firstly, if I am to understand things correctly. Roxas is a special nobody because he did have a heart; he had Ventus' heart. I believe that this is why he could feel things and also why he didn't remember anything from being his somebody. Also; like the other Org members that came back; he also started to grow a heart while he was a Nobody. He just understood that Sora needed to come back, so he sacrificed himself so that Sora could wake up. I don't think this is confirmed in an interview; but I always felt that Sora was able to wake up again because he had Ven's heart back, and that's why Roxas was truly needed.
Secondly; In DDD there is the hint to the fact Sora may have the power to bring people back who have a connection to his heart. This was addressed by DiZ and Riku on the beach at the ending. I think this will play a part in at least Ventus and, due to the previous encounter Sora had with Roxas in TWTNW (Where Sora INSISTED upon Roxas being his own separate person, and desperately wanted to find a way to bring him back) I strongly believe Sora will find a way to bring Roxas back.
Lastly; If you think about it, that's the sort of person Sora is. He is constantly trying to fix everyone, and make sure they are happy. After the vulcan mind meld Roxas did to Sora, I believe Sora is immensely inspired to bring him back. The first game had him single mindedly focused on rescuing Kairi, to the point he ignored Riku. The second game, he was single mindedly focused on finding Riku, to the point he gave up the chance to go back to Destiny Islands at the end of KHI. Sora now knows about the other people who are out there, connected to him and hurting. He will single mindedly try to save them all, to the point were he is oblivious to the danger he is getting himself into. This was proven in DDD when he ran after Riku and Kairi/Terra and Aqua, despite hearing Riku yelling at him to stop.
Sora will find a way to bring him back; just like he'll find a way to bring everyone else back. It's who he is as a person and that's why we love him.