Meh... Way things are heading I'd be fine with them cutting it at 3 seasons. I don't need more of the Belmonts playing second fiddle to Alucard. Not interested.
I at least want a season that covers Old Man Christopher Belmont, since ol' Chris has always gotten the short end of the stick when it comes to the actual narrative of his story.
Hell, finish the show with a season that hits up Simon's Quest. First episode or two can cover the events of the first Castlevania, and at the end, we can finish the series with that iconic shot of Simon kneeling at Dracula's grave, a final end to the nightmare (in that timeline, at least).
One thing I DEFINITELY DO NOT WANT is more focus on the Castlevania 3 cast beyond Trevor's involvement in Curse of Darkness, since it seems like we'll be getting a version of that next.