It depends on if you believe that Ventus is just as emotionally strong as Aqua. Aqua nearly gave herself up to the darkness, but her spirit was so strong that it summoned her friends' Keyblades to help her.
Also, being that Aqua is the successor to Eraqus and therefore the only one who knew how to create Castle Oblivion, not to mention being the only one to know how to find her way back to the Chamber of Repose...well, it would be really bad if she was the one in an eternal sleep.
So, then, we can presume that Castle Oblivion would never have been created, and Aqua would either have been hidden somewhere else or left in the care of Yen Sid or Mickey. Ventus, if he followed Aqua's story, would try to fight Terranort...but would he be strong enough to beat him, thereby opening the portal to the Realm of Darkness? And would he be strong enough, emotionally or physically, to sacrifice himself to save Terra? Would he even know how to donate his Keyblade Armor to such a cause?
Long story short, a lot would change, but we have no way of knowing exactly how much because we haven't seen Ventus thrown into these situations.