I was thinking about the original heroes.
We generally accept that Kris is substituting for Vessel as the cage and Susie is substituting for Noelle as hope girl, but we don't really have a good candidate for Ralsei. "The prince alone in deepest dark." It could be he was the original third hero but his origin is clearly tied to Kris, which makes it very unlikely.

A character alone in deepest dark really only brings to mind the unused dialogue voice that's been trapped in pitch black silence. Seemingly a lightner who fell into the dark an unknown time ago. It can really only be Dess based on the information we have now. But obviously she's not a prince. And the only other prince we have is Lancer who isn't alone and probably isn't in "deepest dark."

We do have a potential prince though. Toriel's dark world form was a queen, Asgore's asylum prophecy glass showed him with a crown, a king, and that suggests Asriel is a potential prince. I think Asriel was supposed to be the one who disappeared into the dark instead of Dess, leaving him to become the third hero. Something happened to switch their places and set us up for the alternate roster we have now.