Cedric_7512
You’ve already established this earth is different from the main continuity, you could take some liberties with the in universe content, make crossovers, or original content that was created in “response” to this new Batman. You could also just have him say it he created it for his own entertainment or to make things slightly more convenient for him.
First of all, love your fanfic. It is hilarious and engaging. You hardly find novels like this. Concerning how to make the Justice League: if we are going by the mc's character you can have a few options: 1. Have the mc make another funny global move that is more outrageous than before , making it the straw the breaks the camels back, so Justice league is formed to find the mastermind. It would be funny to see the mc's reaction of him laughing at this. 2. You make it that as a consequence of the swapping of earths the crisis energy was absorbed by the 'budding villains' so they have some connections with other versions of themselves in the multiverse which can make them stronger than the original selves overwhelming the other heros since the mc, while impressed, still finds it a nuisance he form the Justice League he can leave these 'cannon fodder' for the heros while he acts behind the scenes. It would also be interesting to see batman's interaction with the heros if he reveals his identity when they gain his trust or when he upgrades their equipment. Of course these are my ideas, please keep up the good work.