Priest on Nightwing would be really good, based on his ideas he posted on his blog.
I'd love Priest on Batman, but...
Yeah. I loved his Justice League run, but it was clear he wanted to do more, and got cut off at the knees. I wouldn't want him to do Batman just for 15 issues and then have to leave. He deserves at least a 50 issue run on Batman, if he did 50 issues on Deathstroke.