If Leon were to become Dracula, the events previous to Lament would've to be different.
Leon, firstly, is not an alchemist. He'd have to manipulate someone (possibly Mathias) in creating a Crimson Stone for him for whatever reason, and that would take a LOT of convincing, considering Mathias only resorted to doing it because he felt betrayed by God. And this is on the remote chance he's able to outsmart Mathias, a genius tactician.
Leon would've to undergo a big trauma, much bigger than Sara's death, to choose to become Dracula. As we learn on the ending of Lament, Leon has a character strong enough to not let her death stray him from the path of good as Elisabetha's death strayed Mathias.
Now, if Mathias became the hero and his bloodline were to hunt Leon after this, then I quite possibly think they would battle in a completelly different way, such as using magic instead of melee. Mathias was not a fighter, but a strategist. Leon was the muscle, and he'd be a quite brawny Dracula.
I can't imagine what would the rest be like.