I do agree with what you say because i believe that since ven's heart merged with sora then when sora became a nobody it got released in a way because roxas is an exact look a like of ventus. but also roxas had to merge back with sora so i think ven's heart was merged back with sora. but i have some problems with my theory cause if roxas was sora's nobody and also vens released heart then what happened to vens memories? Somewhere along the way when ven and sora's hearts merged together ven must have lost his memories or have them hidden to deep inside sora's heart.
As for Terra, he wouldn't have a body if he came back because his body was taken over by MX and his soul went into his armor which stayed in the keyblade graveyard. Also his body was eventually destroyed when Xehanort lost his heart to the darkness and became the heartless ansem from the first game and the nobody xemnes in the second. and both beings were destroyed by sora so if terra came back he essentially wouldnt have a pysical body anymore.
I also believe that in KH3 whenever they make it, sora and the others are gonna have to learn about what happened in the past somehow and somehow they have to figure out that ven's heart is in his. Which means he'll want to give it back so he'll have to find the hidden chamber of Waking that axel was looking for in castle oblivion cause that's where ven's body was placed after the final battle. Plus they have to somehow free Aqua. All in all it's a very very very confusing plot now lol.
