I am fond of using the CSA as the reason that the DCCU JL get's together. It would be something not really seen on the big screen in a big tentpole action adventure film, the "MIRROR, MIRROR" evil duplicate trope and all. It also would introduce the audience to a cornerstone concept of the DC universe, that of the multiverse. That could lead to possibilities down the road for many future projects. Basically you have the CSA established as the reason their own world is dying (resource depletion, conflict with other villains on their earth, ect.) and now they plan to take over another world and leave theirs to die. That's more than a big enough collection of power to warrant Superman, or whom ever to bring our heroes together. Plus, since the CSA is made up of "evil twins" it helps to define the Justice Leaguers all the more. You get to do things like contrast Power Ring's cowardly nature against Hal Jordan's fearlessness (just as an example.) Plus the action that you could pull off with that group against the League on the big screen would be amazing. I think the JL:Crisis on Two Earths DTV is a pretty good capabilities presentation for such an idea, as it were.