My absolute pie-in-the-sky dream choice to direct The Flash would be Bong Joon-Ho.
Memories of Murder. The Host. Mother. Snowpiercer.  All excellent.  As much as I love the character of The Flash, I'm wary of a film even being made with how much I love the TV series, but the one thing that would help sell it for me and get me fully excited and onboard would be attaching a spectacular director who would be guaranteed to make a great film, and Joon-Ho would be that guy.  He has shown a skill to do compelling, accessible genre fare, while also having a weird, idiosyncratic, individual quality to them.