Roxas obviously has a large role in KH3, with Nomura two days ago* stating,"Roxas is the utmost secret for Kingdom Hearts III. He's the top secret actually for Kingdom Hearts III so I really can't say anything about him." When we see Xemnas addressing Sora in Twilight Town he tells him that the only way to free Roxas is to open himself up to Darkness. This honestly isn't that huge of a reveal, seeing has Roxas was created when Sora unlocked his heart in KH1 and became a Heartless temporarily.
So it seems that Sora will, in order to bring Roxas back, will unlock his heart again in some way to achieve it,intentional or not. I think this only begs to ask why Roxas is so central to this part of the story, and seeing as Vanitus will be hunting down Sora for his other half I think that the two, being Roxas and Ventius, will be extracted from Sora at the same time.Everything seems to be leading up to that, but it's all just speculation at this point. Please let me know what you think!
Edited by Fullmetal Keyblade, 14 June 2018 - 10:55 AM.