Morrison has said he's got some 'cinematic' ideas for the Flash, that he'd like to turn into an Earth One book. Given his general success in the medium, and the amount of love and respect Didio and Co have for him (rightfully so), I expect that this is who they are waiting for, and that they'll wait however long it might take. I will, too.
But in terms of 'perfect fit', I've said before that I think it's Warren Ellis for his percussive sense of timing and his 'not-so-but-kinda-real world' science fiction sensibility.
I also think Matt Fraction would do a nice job, for what it's worth. And John Hickman. And Tom King.