Perhaps the last man on earth is Superman, but he has lost his powers due to some strange phenomenon. He then has to rely on his wits and the remnants of his Kryptonian knowledge to figure out how to reverse the situation and save the world. He might also encounter some new, powerful enemies that he has to face without his super - strength.
One idea could be that the last man on earth in this scenario is Batman. He has to use all his skills and gadgets to survive in a world full of unknown threats. Maybe he discovers a hidden alien technology that could potentially bring back his fellow Justice League members.
I think Batman has a good chance. He's a master strategist and is used to working alone. He has a wide array of gadgets and his detective skills would be very useful in a post - apocalyptic world. He could also use his knowledge of Gotham's underworld to survive.
One idea could be a crossover where the Young Justice team has to team up with the Justice League to face an interdimensional threat. Maybe a powerful villain from another dimension is trying to invade Earth and the combined skills and powers of both groups are needed to stop it.
Maybe a story where some of the younger members of the Young Justice team are frustrated with the Justice League's methods and decide to take on a mission on their own, but end up getting in over their heads and have to call in the League for help. This could lead to a lot of character development and some interesting dynamics between the two groups.
One idea could be that the Young Justice team gets accidentally transported to an alternate dimension where the Justice League has turned evil. The Young Justice members then have to figure out how to reverse the situation while also evading capture from their once - heroes. Another could be a time - travel plot where the Young Justice team travels back in time and teams up with the early days of the Justice League to stop a threat that was thought to be long gone but has resurfaced in their own time. A third idea is that a powerful new villain emerges who can only be defeated by a combined effort of the Young Justice and Justice League, but first they have to overcome their differences in fighting styles and leadership.
One idea could be a misunderstanding that leads to a battle. Maybe a villain manipulates the situation so that Young Justice thinks the Justice League has turned evil. For example, the villain uses mind - control technology on some of the League members and Young Justice has to fight to save them.
One idea could be a misunderstanding that leads to a fight. Maybe the Justice League mistakes some of Young Justice's actions as being too reckless and end up in a battle. Then they realize they were wrong when a real threat emerges and they have to work together.
One idea could be that the Justice League members gain new gaming - related powers. For example, Superman could have the ability to instantly analyze any opponent's weaknesses like a game character stats screen. Batman could use his detective skills to hack into virtual reality systems within the story's universe.
One idea could be a story where Batman has to lead the Justice League in a fight against a new, powerful and mysterious villain that can manipulate time. Another could be a fanfiction exploring the relationships within the Justice League when they are stranded on an uninhabited planet and have to rely on each other more than ever.
Perhaps in the fanfiction, there is a parallel universe where the Justice League are all vampires. They are trying to protect both the vampire and human populations from a greater evil that threatens to destroy both. They face challenges in getting the human - dominated Justice League in our universe to trust them and work together.
Maybe in the fanfiction, a new gamer is suddenly given the power to control the Justice League in a game, but they soon realize that their actions in the game are having real - world consequences, and they must team up with the heroes to set things right. This could lead to some really interesting character interactions as the inexperienced gamer tries to communicate with the heroes.