I just hope if Aerith makes a cameo in KH3 they replace her with someone else. It'd be great if Mandy Moore came back, and David Boreanaz back as Leon.
As Sign has already said, Andrea Bowen voices Aerith now and has done so for Crisis Core & Dissidia 012 so she'd be the go-to. Doug Erholtz is a fine voice actor and has voiced Squall in all of his appearances since KH2. Again, he'd be the go-to and it's been well over twelve years since either David Boreanaz or Mandy Moore have played these characters.