Somehow, Castle Doran being used as a Fangire HQ for the Checkmate Four in the past leads me to this curiousity.
Where is Kiva in 86, when there's supposedly one in the past? IXA was made to counteract THAT Kiva, and that was it's primary purpose, wasn't it? Remember that Maya had Kivat the II with her when she was first introduced. Otoya possessing Wataru confirms this with his chat with the current Kivat. As far as I also know, only Kivat can activate Kiva's initial henshin powerup.
Castle Doran belongs to Kiva, as far as I'm concerned. When it hides, it leaves behind Kiva's symbol in the buildings it hides in, and Tatsulot laid sleeping inside the castle, ready to be awakened when it was time for Kiva to shed into it's true form. So for Castle Doran to be used in 86 as it is, I wonder now if the 86 Kiva was the original King.
Because of the events we still have yet to know, Kiva, instead of being passed on to Taiga, is passed onto Wataru instead. If that's true, then perhaps this new Rider is a creation to replace and perhaps even surpass Kiva, who is now set with the task to kill fangire. It also explains why Castle Doran is no longer a Fangire HQ. Something happened in the past, probably involving Otoya, the Arms Monsters, and perhaps even Maya herself, for the current events to be how they are.
Hell, I wouldn't be surprised if the Bloody Rose, the violin his father made, somehow reprograms Kiva to kill Fangires instead of supporting and ruling them as King. Would Maya have had anything to do with this, I wonder?