When I first went into R/R, like the very first time, I assumed we would actually be playing as Riku Replica, because it seemed at the end of the game like they were planning on making him a more substantial character. I wasn't really shaken from that conviction until the two Rikus outright faced off, even Vexen didn't convince me at first.
I was kind of let down that we wouldn't have any epic Riku vs Sora fights from Riku's perspective... and still haven't, come to think of it.
All of Reverse/Rebirth was filled with wasted potential. Ansem, Maleficent and even Zexion explicitly state that the only things/people that Riku could see were dark beings. Why couldn't we have any of them interacting with Riku, then? Yeah, they're just memories, but they could have brought in a very interesting insight on how Riku viewed his journeys throughout the first game. How often did he interact with Oogie Boogie? Did he ever even speak to Maleficent? Did he ever lock blades with Cloud whilst the latter was still under Hades' employ? Hell, they could have easily BS'd their way into shoehorning a Sephiroth battle in there somewhere. The memory versions of the worlds act similarly to the Pensieve from Harry Potter anyways, so why not just say that he was somewhere where Riku couldn't really see him or something, IDK.
I'm also still really bummed out that none of the other Organization members even spoke to him at all. The basement dwellers barely interacted with him, so it would have been nice to see Axel, Larxene and Marluxia at least keeping tabs on what the hell is going on personally instead of just using their crystal ball. I just feel cheated out of all the story potential and boss fights that we missed out from on due to the narrative structure of the game.