Hmmm, fitting it all together? In a fanfic-type of way, you could do it like this(but remember, this is only with the info we have RIGHT NOW, because any DLC or additional stories could mix it up):
1. Gabriel is father of both Mathias and Leon, but with different women. Percautions were made with Leon to keep him from being born as a Dhampyr(if Gabriel had become a vampire by then). Both inherit half of their father's powers(Mathias inherits darkness, Leon inherits light). Leon's raised as a Belmont, his mother is a Baroness. Mathias raised as a Cronqvist, learning the dark arts of Alchemy, creating the Crimson Stone and becoming allies with Death.
2. Rinaldo Gandolfi in LoS is LoI's Rinaldo's father(LoI is Rinaldo Gandolfi Jr.). Rinaldo Sr. teaches his son the art of Alchemy.
3. Gabriel, himself, becomes Dracul, and disappears, while his older son, Mathias, befriends Leon, and the two are unaware that they are, in fact, half-brothers. The enter the military together and prove lead their men to victory. Prior to this, Mathias marries Elisabetha, and Leon courts Sara.
4. Elisabetha dies, Mathias goes crazy, and contructs a way to scorn God. Tricks Leon into killing Walter, then becomes a powerful vampire. Mathias leaves Leon to fight Death. Leon becomes the first "Belmont" of the "Vampire Hunting lineage". Mathias goes off into the world, building his nation, and eventually goes to seek out the father who abandoned him.
5. Mathias meets Gabriel, now under the name Dracul, and is turned away(Dracul wishes to be left alone). But realizing that he is his father's son nonetheless, he sheds his name as Mathias Cronqvist, and dons the name of Dracula(Son of Dracul, the Dragon).
6. Hundreds of years later, he rules his domain in Romania as Dracula Vlad Tepes, falls in love with another woman(Lisa), who bares him a child(Alucard, Adrien Tepes), but is burned to death years later from being a witch. Dracula turns on humanity, until Trevor kills him(with the help of Sypha, Grant and Alucard).
-Events of classic canon-
1999- Zobek seeks out Dracul, knowing that Satan's resurrection is at hand. The two decide to seek out Dracula(Dracul's son), who's become super powerful, to help them. Dracula's hunger for power pushes him into becoming the "Master" of Satan himself(because Dracula's the Dark Lord, and all monsters and demons will sooner or later fall under his command). Gaining powers over Satan himself, Dracula innitiates the Demon Castle War, attempting to raise Hell on Earth. Julius is sought out as the world's final hope.