Before anyone gets on my for bashing everyone's theories before, NOW I won't mind them as much because we actually have a trailer and information to actually base some theories on.
Anyway, so WHAT IF the "my blood" guy is Alex Wesker?
He did he had super human abilities. He did strong melee attacks, moves quickly, I believe it was him holding onto a helicopter, and he jumped down a very high distance. This would suggest he has the same powers from the same experimental virus Albert had. (And as we know he is the only other Wekser child.)
We also see his friend inject him with something. This could be the same thing Albert needed to stay regulated.
So we also know Alex found the secret to immortality, and working with viruses and such, its not far fatched to assume immortality could be given from a syrum via injection. Thus, this would make his blood important.
So on why the bad guys need his blood, it could simple be Alex ISN'T a bad guy. He was just a researcher working for Spencer who BETRAYED him. So maybe he realized he should be using his skills and discovery for good?
So this would all suggest Alex was the one who started the prject AND participated it in. Not two different people as some people thought due to how unclear it was.
But that would also mean Alex is OLD. To be a researcher, he must AT LEAST be like in his 20s. I would assume late 30s to be a head researcher. If he selected CHILDREN, that means he's a generation older than all of them. Albert was 48 in RE5 (2009), so that would mean Alex is like, 70 something o.0.
Solution? His findings just make him younger. Not immortal like grow old and never die. Just an interesting thought to bring up.