Uhm... I was under the impression that Rondo in Japan was called Dracula X because it was the tenth game in the Akumajou Dracula series.
So a potential Castlevania V at this point would have a redundant title, wouldn't it?
1. Castlevania/Akumajou Dracula
2. Vampire Killer/Akumajou Dracula
3. Castlevania II: Simon's Quest/Dracula II: Noroi no Fuin
4. Haunted Castle/Akumajou Dracula
5. Castlevania: The Adventure/Dracula Densetsu
6. Castlevania III: Dracula's Curse/Akumajou Densetsu
7. Super Castlevania IV/Akumajou Dracula
8. Castlevania II: Belmont's Revenge/Dracula Densetsu II
9. Akumajou Dracula X68000 (released later as Chronicles on PSone)
10. Castlevania: Rondo of Blood/Akumajou Dracula X: Chi no Rondo
While a majority of those are remakes, and it still stands that Rondo was the tenth game in the series, thus leading to the X, which is (as you all know) Roman numeral for ten.