Within a different universe, different characters exist together. However, it can come at a cost. Meaning that even if there are good guys, bad guys will always be around to team up against them. No matter what world they're from, they would always find ways to team up against each other. Such as evil armies marching together, evil leaders coming up with plans to together, having things in common, etc.
Heroes on the other hand, do the same to stop their enemies. However, it can be complicated, as it varies depending on who they are. As of now, society has now fallen due to them having been taken out by an evil faction known as the New Imperial Nation. It is an evil nation led by 5 villains, being known together as the Imperial 5. Their names were Morag, Kuvira, Smaug, Miles Quaritch and Queen Ima. Morag is a former castle keeper, Kuvira is an earthbender, Smaug is a dragon, Miles Quaritch is Na'vi Recom, and Queen Ima is an Egyptian Queen from a prehistoric world.
The huge reason why they teamed up was because they want to take over the world. They had also made allies with other villains too. And of course, Morag is the only one to not have an army until she was able to rally enough villains to be an army. They made allies with other evil factions like the Atrox Army, SUSPENSE (Secret Underground Society of Peach Entrepreneurs Nixing Sour Exports), Chimera Corp, Kraken Terrorists, Incognito Labs, etc. Even teaming up with individuals and hostile species like Dr. Carol Linnaeus, Chandler McCann, orcs, great spiders, etc.
Together, they were able to take down many factions with ease. That included them having also made machines or bioweapons in addition to their armies too. While many escape their wraiths, others tried to fight back, but never made it back alive. Those of them who had not participated in the fighting, went into hiding. All existing in small pockets elsewhere in communities.
During that time, they also traded with whatever supplies they had. Not all though as others will do anything to survive like raiding, pillaging, killing etc. All of that is all driven in a world where it's now post-apocalyptic. Especially as the villains have established areas to meet their needs. As of now, the only hope would be is for a hero to help fight back against the villains.