I don't think the overall story of CV is getting stale at all, since each game is basically self-contained. Like you said I think the era that has vast room for potential is the 300 years that Dracula was dormant. After the events of CVIII, there's not much to tell in detail since Dracula isn't allowed to survive for more than 1 night (however, there are many questions to be answered about Dracula's whereabouts between CVA & Belmont's Revenge). That's what I like about most of the games; the plots are short and sweet, they don't really require epic storytelling. LoS will have an epic story, though. I still see where you're coming from, future games (preferably Metroidvanias) could expand on the dialogue & characters a bit. OoE had this, but it seemed artificial. Where were the other members of Ecclesia? How did Barlowe receive the knowledge of Dominus? etc.
The other tidbits you mentioned, can possibly be covered in radio dramas, mangas, things like that.
Well here are some of the plot holes Im sure the wizard is talkin about here.
First off Lament of Innocence takes place in 1094 and states that Mathias (Dracula) is 32 which would place his birth roughly around 1062. SotN contradicts this because it states Dracula is around 800 which would mean he would be born 997.
EDIT after further investigation of Rondo and Dracula x both also state that Dracula is over 800 years of age.
EDIT Also worth noting is that in The Dracula X Chronicles the manual for both Rondo of Blood and Symphony of the Night neglect to mention the age of both Dracula and Alucard though still mention Richter to be 19 and Maria 12. So there is a chance that the age mentioned in both manual was change so not to contradict Lament of Innocence.
EDIT: Also SotN takes place in 1797 and states that Alucard is roughly around 400 placing his birth around 1397. Lisa was executed in 1470 now if she had a normal human life's span that would mean she gave birth to Alucard in her 20s making her birth roughly around 1377, if she did get executed in 1470 that would place her age around 93 even though Alucard remembered her as being youthful.
EDIT In hindsight her being 93 when she died could've been the give away that prompted Alucard to realize it was an illusion despite that the succubus made a vital mistake in telling Alucard to get revenge for his mother. If the succubus did get her appearance correct then her age would've been no more then her 40s placing her death around 1417 and making Alucard around his 20s at the time of his mother's death. If Legends was included it took place in 1450 making her age at the time of her death roughly 53.
Second in CV3 Dracula is defeated in 1476 thus forcing him into a 100 year sleep to regain his powers. CV adventure takes place in 1576, 100 years after his defeat and he is again defeated BUT not killed because it was also revealed in Belmont's Revenge that Dracula managed to survive and bide his time until he had regained his strength but is finally put down again by Christopher in 1591( Read the manual for the details of how Dracula survived and his whereabouts). Now Castlevania takes place in 1691 meaning that when Dracula was defeated in 1591 it would have forced him to sleep for another 100 years thus taking place in 1691. If this is the case then Curse of Darkness which takes place in 1479 he shouldn't have returned, since Hector defeated him, until 1579 three years after Adventure takes place, thus contradicting the 100 year resting cycle since Dracula seems to resurrect exactly 100 years after his death, Dracula's Curse(1476) and CV Adventure (1576) prove this fact as does Belmont's Revenge (1591) and Castlevania 1691 since Dracula technically did not die until 1591 when Christopher defeated him.
Finally the inclusion of Bram's Novel (I'm assuming they got the rights to say that Quincy was the descendant of the Belmonts being that the Stoker family got pissed when Nosferatu came out in the 20s) Which takes place in 1897, now I can't remember if Bram actually stated Dracula's age but he did use the actual Vlad Tepes which going off of the novel would actually contradict Dracula's entire history being that Vlad was born c. 1431.
I wont even go into the 1800s though
my own speculation is this. Nostraduamus predicted the whole end of Dracula thing so as time grew closer I'm sure Death heard of this prediction and believed it and thus made several attempts at restoring Dracula so that they could change the events predicted.
So in conclusion yes there are plot holes.
Also I thought the japanese were good at math......or is that being too sterotypical?