The reason why I would like to see Rock is more for consistency and completeness since they are going back to the events of the original SoulCalibur for the Dreamcast. It's similar to how Capcom added Dee Jay and T.Hawk to Super Street Fighter IV even though they weren't really popular characters but instead because they were in Street Fighter II.
Another reason why I want to see Rock is the lack of "big man" with Astaroth being the only one. I would like to see another grappler or big man added to the game. He also uses a different weapon type which is a warhammer instead of an axe.
Lastly, if Rock is to be added, we could be seeing a completely revamped character. Rather than dwelling on his failure as a character in the past, think about the possibilities that Namco could do with him in the future. Seong Mi-na is made to be different from Kilik, so I don't see why they can't do the same for Rock. Although he is a "returning" character, my hope is to see a revamped character that feels fresh and new. It's also similar to how I would like to see Li Long in the future: revamped with a three-section staff instead of using nunchucks.