My issue with using Wally West in a movie, is that Wally West is fundamentally defined by his status as a legacy hero. While this doesn't mean you have to do a bunch of Barry Allen movies before using him, it does mean that you have to be comfortable writing Wally with a relationship to his posthumous predecessor, in a world where such a posthumous predecessor is possible and sensical. I remain dubious that WB would be able or willing to do such a thing well. They'd either strip out everything that makes Wally a distinct character ala Cyborg ( bad ), or utterly botch the "the world existed before this movie and this hero" narrative aspect ( bad ), or likely both ( bad ).
Which is a pity, because I like Wally West much better than Barry Allen, at least in the comics. Given my druthers I *would* make Wally West the Flash in the movies, with the Barry Allen Flash part of the prior generation of heroes to the current one. But I'd rather a good Barry Allen movie than a bad botched Wally West movie.