He left because no one at WB cared enough to listen to what he was doing...I look at any DC adaptation from a grown ups prespective...the chances of 1 Flash film are slim enough, much less a trilogy or on-going franchise......
It's not like Warner Bros. will sit down and say "A trilogy of Flash movies is too expensive. We're only going to make one." They should approach Flash as a medium-budget summer blockbuster (by medium budget, I mean in the vicinity of 80-100 million) with a self contained story that doesn't leave too many loose ends, but has the potential for sequels. If it's a hit (which it will be if it's well made, and has a good marketing campaign), then they can start developing two sequels. It's movie business 101.
The thing that is always a pain with Flash however, is that even though he's a second tier hero, his rogues gallery is on par with that of a third tier. Most of his villains are very lame, and finding one that would work well as his primary antagonist would be pretty hard. Zoom is one of the only Flash villains who can give him serious trouble and adds something to his character, though I'd be a bit leery of using an "evil version of hero" villain right from the first movie, because that sends the message that the hero has a shallow rogue's gallery (which Flash does, but they should at least TRY to make up for it).
However, the success of Iron Man shows that an "evil version of hero" villain can still be used to launch a franchise as long as the story packs a serious punch. As long as they give Zoom his own freakin' costume instead of giving him an inverted Flash costume, I think he could work OK as the main villain as long as there are plenty of other thugs for Flash to go through in the movie first. The problem after that though, is where do you go for the sequels? I haven't read it, but the Cicada story line sounds like it's quite interesting and could serve a movie well-- the idea of a cult justifying human sacrifice by claiming the people would have died if not for the Superhero who saved them is intriguing, and it could give Flash a solid emotional burden to deal with throughout the film (and it's nice and dark, which Warner Bros. likes, apparently). Once you get past Zoom and Cicada though, you're down to Gorilla Grodd, Captain Boomerang, Captain Cold, and Mirror Master... And frankly, I'd have a pretty hard time imagining a movie with any of them as the antagonist, so they'd either have to seriously overhaul one of them to be a real threat and contribute to the story, or they'd have to team up several of them.