Imagine this,
Mathias Playable initially to help Leon, but in reality to Kill Death, As he and Death come to an agreement, Mathias still defeats him to gain dominance over death, leaving only his rebellion to God left, which he succeeds in becoming a vampire.
The Vampire Killer Ritual could involve blood, Maybe Sara should bite Leon at the moment she is killed, It would explain why only Belmont blood would draw the whip's power and at the same time give an explanation to The Belmont's supernatural feats.
Rinaldo would interact with both Leon and Mathias, Mathias he would initally treat with full respect and slowly see his turning.
A Mathias Campaign could also give tidbits to his feelings towards Elizabetha, Maybe something along the lines of during the crusades, all Mathias asked from God was to keep Elizabetha safe, after so much killing (or whatnot) in the name of God, just time find his dearly beloved has fallen.
There could be a DMC3 style showdown between Dracula and Leon where the outcome would obviously result in the Castlevania Legacy.
Also, Since they were on Common grounds, Leon's choice to Hunt the night with his bloodline would have a stronger influence if the betrayal was more betrayal-y. Example, Mathias could have saved Sara at one point, but instead wanted Leon to lose his loved one so he could "feel his pain".
Lastly, I think that Leon should have a child, not during the game, but at the epilogue, That way the Betrayal is deeper, and the child would grow up hunting the night, for none should repeat Leon and Son's Tragedy.
All LoI really needs is a bit more writing into it to be a satisfying Origins story. Since IGA is more about the Game first, then story, I think getting some help on story would have been a good idea.
what do you guys think?