Re: Which villain(s) should the reboot start with?
Oh, they'd probably want to do top-tier. That puts a lot out of the running like Killer Croc or Black Mask. And it would either be a new one for live-action or one they haven't done in a long time.
The Riddler does seem likely, mostly because from what I have heard the producers of the sequel to the Dark Knight did indeed push some of the first-draft screenwriters and story writers (David S. Goyer?) towards incorporating him as the main villain in that one. Obviously it went nowhere, but that they'd push him again seems likely.
Mr. Freeze would be great, to finally get a serious version of him. He has a sympathetic enough backstory that it provides emotional resonance, AND the super science bent would set the tone for a new series that is more comic book-like and moving away from the "realism" of the Nolanverse.