They could make the Tower of Babel into a crossover and have Justice Society to be formed by the new heroes in the end of the crossover.
However, which villain can steal Oliver's plans? Merlyn, maybe? He can form a small team of villains like the animated movie that adapted Tower of Babel and try to conquer the world after he defeats the heroes. So, it will be a world-threatening threat and not just Green Arrow's problem.
Combine Tower of Babel with Justice Lords. It could work out something like this:
On
Earth-50, Lex Luthor has become president. He consolidated his power and was able to declare war on aliens and meta-humans, culminating with the public execution of Superman. This led to J'onn and Oliver gathering up their world's heroes and taking over the country in order to stop Lex.
Somehow, the Justice Lords end up on Earth-38, and attempt to purge the Luthor line there, as well. Kara brings Lena to Earth-1 to hide her, but is followed by one of the Justice Lords. One by one, heroes start being taken down (either Legends or guest like Mari and the Hawks, non lethal attacks but serious nonetheless) until Oliver stops the attack on Barry. Asked how he knew what the assailant was going t do, Oliver replies "Because he was using my plans."