I'm serious, so many Batman supporting characters, including ones with currently no Mainstream recognizably, have carried their own spin off titles. There have been times when between a 4th to a 3rd of all monthly DC comics being published in the main continuity are set in Gotham city (or nearby Bludhaven). More Comics then marvel usually has about the core Avengers team members (X-Men is in fact Marvel's biggest franchise, which Fox is demonstrating by making a Shared Universe of just them).
Start with a Batman solo movie where he works on his own, even though the others already exist, not unlike the first two Arkham Games. Though Oracle can contribute and maybe at least a small appearance from Robin.
Then do movies for Nightwing, Tim Drake Robin, Cassandra Cain Batgirl (Spoiler would have a supporting role in both) then movies for Birds of Prey, Catwoman and Batwoman. Then the team up movie can be called something like Batman: Gotham Knights. Maybe have two Solo Batman movies for each phase so we don't forget he's the star.
That's how much bigger DC's universe is, Marvel could only do that with X-Men. DC's biggest is Batman, but at times Superman can be similar when you have Supergirl and Superboy, and some of the local Metropolis non powered Heroes like Steel, Gaurdian and Gangbuster, also Rose and Thorn.
And DC could theoretically do it with Green Lantern too, have a Saga about Earth's Sector's current GL, and then one or two about the Corps out in Deep space (Don't forget there is a Squirrel GL who can rival GOTG's Racoon). then throw in The Corps unaffiliated GLs like Alan Scott, and Guy Gardner back when he had a Yellow Ring.
Recently I heard well over half of all DC's comics where those 3 franchises together. I just wish WW and Flash could grow that much, people love Diana's teen Sidekicks too, but they never get their own Serieses, just supporting roles in WW and Teen Titans.
WB isn't gonna do that anytime soon, but when the current Cinematic Universe proves profitable, I think they may consider it 10 or so years down the round when it's time to Reboot again.
But back specifically to the Batman shared Universe idea, I think that'd ideally be done with a TV Show format actually. For the basic Fall-Spring Season have several shows running (the titles I listed above, plus a Gotham Central Police Procedural), literally more or less make a brand new Network just for Batman shows. Then during each Summer between the seasons have a special miniseries for the Big Crossover events.
But Warner will never do that, because the Big Screen always get first dibs.