I don't know how it would work, politics and all, but i would love a human war, taking place - terrorists, with threats posted around the globe, the terrorist gave the government/s an ultimatum and literally it was down to JLA to stop it - i would love to see a bigger scale version of the Smallville battle, with flash, batman, GL all involved as two countries go head to head - the JLA want to stop the battle, but also help/save lives.
I need to properly think about it, but a country in the middle east could be sponsored behind closed doors (by lexcorp), to attack america if Superman and the JLA do not hand themselves over or agree to not interfere in public relations (even though they wouldn't) - civil war could be on the verge as the nation gets worried, other countries are ready for war, to help out allies.
JLA and investigative reporters

realise it's a scam and try to reveal it is luthor before too late.
I just feel, in the real world, if america suddenly had 7 meta humans/aliens living there, other nations will be like WTF - is it second coming, a super army etc. Also, i feel it needs a grounded movie to be different to marvel.
I use Transformers 1 as an example, i loved the tone at the start, where the army were attacked by a decepticon - it was totally believable, considering it was an alien attack.