One idea could be exploring how the remaining characters rebuild society in a world forever changed by the 3rd Impact. Maybe they have to deal with new forms of life that emerged from the cataclysm.
Definitely Shinji. He was at the center of so much in the original series, and after the 3rd Impact, his character development would be crucial to explore. He could be a broken, shell - of - a - person trying to find meaning again, or he could have newfound strength and purpose.
One idea could be a story about a new Valkyrie who has a unique power that she struggles to control at first. For example, she might be able to manipulate time on a small scale, but it causes chaos in battles until she learns to master it.
One plot idea could be that the Dragon Balls are accidentally brought into the Evangelion universe. This causes a stir as different factions in the Evangelion world, like NERV and SEELE, try to get their hands on them for their own purposes. Meanwhile, the Z - Fighters follow the Dragon Balls and end up in this strange new world. Another idea is that an Eva unit gains the ability to use ki, like a Saiyan. This Eva then goes rogue and starts causing chaos, and it's up to both the Evangelion pilots and the Z - Fighters to stop it.
A possible plot idea is that Godzilla is drawn to the Geo - Front in Evangelion. The reason could be the powerful energy sources there. The NERV organization is then forced to either work with Godzilla or find a way to get rid of it. They might study Godzilla's biology to see if they can use any of its abilities to enhance the Evas. Another plot could be that an Angel takes on the form of Godzilla to deceive the Eva pilots. This would make the pilots question their usual tactics when facing an 'Angel' as Godzilla has different strengths and weaknesses compared to the normal Angels. Also, a plot where Godzilla's presence causes a shift in the power dynamics between the different factions in Evangelion, like SEELE and NERV, could be really interesting.
Well, a cool plot might involve Goku and Shinji Ikari swapping places for a while. Goku, with his positive attitude, could have a big impact on the Evangelion world, while Shinji in the Dragon Ball Z world would be forced to learn about ki and fighting in a very different way than he's used to. This could lead to some really interesting character development for both.
One idea could be exploring the new generation at Hogwarts. Maybe a character discovers a hidden spell that was forgotten during the main series and it has unexpected consequences for the wizarding world. Another could be about the rebuilding of the wizarding community after the war, focusing on how different families reunite and deal with the losses they've suffered.
One idea could be a story where the Mandalorian has to team up with an old enemy to fight a new, greater threat. Maybe a powerful Sith Lord emerges and they have to put aside their differences.
One idea could be exploring a different origin for Toothless. Maybe he was part of an ancient dragon tribe that was thought to be extinct. Another could be a story where Hiccup discovers a new dragon species that has unique abilities and forms an alliance with them. A third idea is a time - travel fan fiction where Hiccup goes back in time to prevent a great dragon war from ever starting.
A great plot for Warframe fan fiction could be a Tenno getting trapped in a parallel dimension where the rules of the Origin System are all messed up. They have to find a way back while dealing with strange versions of familiar enemies. Or perhaps a story about a Tenno who discovers a hidden Orokin vault that contains not only powerful weapons but also a secret that could change the balance of power in the Origin System. Also, a plot about a Tenno's struggle to control a newly acquired and extremely powerful Warframe that has a mind of its own could be really engaging.
Well, you could have a survivor fan fiction where a contestant has a hidden skill, like being able to communicate with animals on the island. This could give them an edge in finding food and avoiding danger. Or perhaps a story where there's a traitor within the group, someone who is secretly working with the show's producers to create more drama. Also, a plot where the survivors find a map that leads to a hidden part of the island with a bountiful supply of food and shelter.