If Reeves is able to make the movie Giacchino's almost certainly a lock. I think he's a tad overrated as a composer, but he tends to make better scores when working with Reeves so maybe he'd do something special with Batman.
That said, my dream pick would Abel Korzeniowski. I think he'd really nail the gothic romance of the character, and it'd be cool to see him do a big budget feature. Christopher Young, Clint Mansell, Debbie Wiseman, Howard Shore, Joe Kraemer or Alexandre Desplat would be interesting, too. Patrick Doyle from the early 90s would've been amazing.