Okay so ever since the trailers dropped, I have an interesting theory on how Roxas will return in kingdom hearts 3. How it comes!:

 

So you know in one of the trailers Ienzo mentioned something about the data twilight town and it's relation to Roxas. What IF, they bring out Roxas's body from the data twilight town assuming it's data too and use the body from the data to bring Roxas back? 

I mean yeah, I think that's what the trailer implied from the beginning, the problem here is if Sora can release Roxas's heart from inside him withouth giving himself to Darkness, and also what if Ven's heart was Roxas's from when he was born? That would complicate things as two entities would be sharing one heart, thus only one can return. Nomura is an evil plan master (no wonder he relates to Xehanort) so we don't know what to expect.

The big gap in your theory is how one would turn data into something tangible. If that were the case, we would probably be seeing data Sora walking around in the real world. Plus, if they were somehow able to even bring data Roxas into reality, then you've already got it solved. But Roxas wouldn't necessarily be "back". The original Roxas would still be trapped in Sora, and now you're left with two sentient Roxas' existing. I really doubt Nomura would deal with the philosophical implications of the equivalent of "killing" a character off by replacing the conscience within data Roxas with the real Roxas. 

I mean yeah, I think that's what the trailer implied from the beginning, the problem here is if Sora can release Roxas's heart from inside him withouth giving himself to Darkness, and also what if Ven's heart was Roxas's from when he was born? That would complicate things as two entities would be sharing one heart, thus only one can return. Nomura is an evil plan master (no wonder he relates to Xehanort) so we don't know what to expect.

I'm pretty sure that Ven's heart only affected Roxas's regarding appearance. Recall that his heart was already broken at that point and was permanently handicapped and was just using Roxas as a host. In time, Roxas most likely grew his own heart while in the organization. Roxas's and Ven's personalities are quite distinct from one another, which is evidence of this. 

This is a very interesting way of looking at things, but remember that the heart is so much more than a system, so how can a heart be able to work on a digital body? I mean, Ansem The Wise spent his life trying to learn the workings of a heart and seeing how it could work and malfunction! Unless...unless perhaps the contents of the bottle he gave Riku at the end of Dream Drop Distance--oh! Since that is the data he collected from Sora from his year long sleep, it could be that this data is the peak of Ansem's research, and like I had speculated before, said data could form the Key To Return Hearts, which I'm sure would play a vital role in bringing back Roxas, Namine and Xion, because they'd be able to have their hearts back!

 

So perhaps the data could simply be an empty host, devoid of Roxas's personality and anything else, and by then adding his heart would the body mesh into existence! If that makes sense, that is. x3