If done right, they could explain pretty much everything what is left open for now.
Because Alucard´s a Vampire, the plot could span over diffrent eras, maybe divided in Chapters. Could go from Post-Symphony (tell why the belmonts mustn´t touch the whip like darkwzrd4 said + maybe show how Al sealed Draculas soul before OoE´s events, because, as far as I know, wasn´t he the one who did it? correct me if I´n wrong) until the battle of 1999, showing the uber-showdown we all wanted to see since AoS.
Beeing a vampire sure has it´s good sides...
But yeah, I said IF it´s done right. The whole thing could end with more questions than answers... again =O