[806mtson]

Though, someone on the Kamen Rider Wiki discovered that the Contract Holders of the Deadmans are basically the antithesis of the Riders which the Vistamps are based upon.

So, if we find a mystery Proto Vistamp and don't know which Rider it's based upon, we could take the personality or the motives of the Deadman's Host and reverse engineer the Rider from that.
Given the cowardly, yet highly intelligent and manipulative personality of the Planarian Deadman's Host, which was used to get a lot of people to join the Deadmans, the Rider the Planarian Vistamp could be based upon has to be a brave and friendly Rider whose whole thing is that they get a lot of people to join the side of the Riders.