Warning : LONG POST AHEAD
I'd probably go the route of a few others and give Mathias a game featuring his descent (or would it be ascent?) into Chaos. Gameplay would most likely be along the lines of Legacy of Kain, but featuring such elements that Blood Omen and Defiance made note of, like blood-drinking to sustain one's health, and having vampiric abilities being more practical in use than mystic (such as Kain having better use of the Mist transformation to pass gateways when he can just as easily evade enemies in its stead).
It would probably feature elements of a full action RPG, such as Skyrim, but be sensible to the timeline; such as travelling to far corners of the world to become more adept in the Black Arts, rather than the implication that Dracula gained them all from Walter/over time. With each successive addition of Dark power, Mathias slowly gains a look, air, and "feel" of hatred and anger about him, subtly altering his mannerisms and movements to illustrate the change to Dracula and the subsequent loss of his humanity.
I would introduce a logical and expanded place for Galamoth and his role in the canon, and give him a part as a final boss or alternate final boss. The Church and all it entails would become prominent, fighting Dracula/Mathias during the Crusades and Dark Ages in a grand act of irony, as God and His agents are Mathias' chieftain enemy. This would also introduce the role of the Belnades as Sypha's ancestors, tying the future of Yoko and the Demon Castle Wars into the mix. Perhaps a hint or two of Dracula taking precautions to ensure his soul would remain even if his body was permanently destroyed? A Castlevanian Horcrux, mayhap? Being a genius tactician, considering and planning for the possibility of his ultimate downfall is very likely. Enchanting his very soul to go into respite, and split to multiple inheritors to ensure his survival is not a big stretch, especially when you've got Death on speed dial. Thus the Dark Lord Candidates and Soma become tied in nicely.
Also included will be journeys into the various Realms of Chaos, and how the Castle itself becomes a sort of living entity that changes over time, like its master. These changes will be justified as side effects of prolonged exposure to the entropic nature of Chaos' power, and thus a reflection of Dracula's soul at the various times of his resurrection. Same goes for the Castle. The "Creature of Chaos taking many incarnations" notion is now more logical.
Lisa and her connection to Elisabetha will be elaborated upon. Should she be her reincarnation, I would make it so that it was an act of the divine in an attempt to bring Dracula back to the light (after all, since God is written to have forgiven much worse than centuries of being called names and defied, it would only make sense that He would pity what Mathias has become and want to restore him to himself by returning his happiness to him how he could). Should she simply be a human woman with coincidental similarities in appearance and kind demeanor, it will be that Mathias' heart awoke within Dracula and rekindled the feelings for Elisabetha anew (since Dracula appears to lose situation memories over time, while the emotions of those memories remain strong), only to have that final tiny shred of humanity be shattered forever when she is burned at the stake.
Shaft will also be given an expanded backstory, as there would surely be one particular cult that would surpass the others in power and willingness of extent to serve Dracula. Shaft could be the leader of this cult, given extended superhuman lifespan due to his adeptness in the Dark Arts. Perhaps he could have a more personal reason to serve beyond "cleansing" the world in Chaos, such as suffering a similar fate as Mathias and taking a similar path in anger and pain. Perhaps he's just batshit nuts. Perhaps both.
Finally, Death's odd role would be made sensical. After all, how can the divine master of life's end truly "die?" My theory is that the form he takes is destroyed and his power and ability for combat and magic wane, while his eternal power of taking life when the sands are gone remains as constant as ever. The effects of the Crimson Stone's apparent power over him will be elaborated on; perhaps, with the Stone's power to completely take one's soul and assimilate that soul's abilities and prowess entirely to the Stone's owner, it has a connection with Death. Perhaps this was a stone created by the divine to balance the power that Death commands? After all, he could theoretically take the soul and life of almost anyone (God may or may not be excluded from this, as physical well-being and age-based immortality can still be ended when the soul leaves the body, but that's a whole different speculation that I will not touch on), so he would likely need a form of a controlling factor, or an equilibrium device of sorts, to prevent him from going rogue and killing everything. The Crimson Stone might contain the same power as the role of Death, but on a more limited scale to still give him the predominant niche.
tl;dr I'd try and tie ERRYTHANG together. Except Curse. That would be retconned to hell and back. jk CoD wasn't that bad
EDIT: Forgot a few notes. Since all of that obviously would span over the centuries, this game would be long as shit. A plus to that, is that you are Dracula in each of the canon games, fighting each generation of Belmonts as they come. Maybe you'd even get a sort of tactical strategy segment where you plan out where enemy waves will position and the like. Just a thought.