Pretty self-explanatory.
Characters and people I'd have liked to see introduced in Castlevania:
Sherlock Holmes:
Dunno exactly why, but I find his character very compelling. Some of his stories include him debunking a supernatural thing and solving the case (like in Scooby-Doo). I think it'd be cool to see how would Sherlock react to the Dracula legend.
Nazis:
With WWII as it's background, PoR was an AMAZING opportunity to introduce a Nazi element as a villainous party. It could be an original character, even. No need to even mention Hitler.
With rumors of the Nazis conducting supernatural experiments, it would be nice to see a Nazi party going after Dracula's powers to win the war and possibly take over the world (and end up wiped out by Dracula himself after noticing the Dark Lord is nobody's pal).
Bram Stoker:
Obviously. The potential for nice writting here is stupendous, but I've already seen stories where Bram Stoker actually witnessed Dracula and wrote about him as a fictious character, and... It didn't turn out very nice.
The premisse is good, though, so I'd be happy to see it executed nicely.
Van Helsing:
Obviously².
More interesting would be imply that Van Helsing is actually related to the Belmonts somehow.
Rasputin:
No real reason beyond "Rasputin is freaking cool."
Oda nobunaga:
It'd be nice to introduce him as a pretender to the Demon King position, because he DID call himself "Demon King" after all.
Maybe he could've been turned into a vampire by selling his soul on his downfall at the Honnoji Temple, and now is seeking to overthrow Dracula? Maybe he did die, and had his soul enslaved by Dracula, only to find a way to rebel? Many interesting possibilities here.