Tomoki is definitely a popular choice. He's the main character, and his relationship with the Angeloids is central to the story. Fan fiction often explores different aspects of his character, like his growth from a perverted teenager to a more responsible person.
Another interesting plot could be that the world of heavens lost property gets merged with a parallel universe where the rules are slightly different. The characters would have to adapt to the new situation, and there could be new villains or challenges. For instance, the Angeloids' powers might work differently, and they would need to figure out how to use them effectively. Tomoki would also have to deal with a version of himself from the other universe, and they could either cooperate or compete depending on the circumstances.
Well, without actually reading it, it's hard to say precisely. But generally, a graphic novel about lost property might revolve around items that are lost and the stories associated with them. It could be about the search for lost items, the people who lost them, and the emotions involved like sadness, hope of getting them back.
In a crossover with 'Fate/stay night', characters like Saber could be included. Her noble and strong - willed nature would contrast with the more carefree and sometimes naive angeloids. They could be on a quest together, with Saber's combat skills and the angeloids' unique powers complementing each other.
You could start by checking your local bookstores. They often have a section dedicated to graphic novels. If they don't have it in stock, they might be able to order it for you.
Overall, the ending of Heaven's Lost Property manga is somewhat bittersweet. There are resolutions to major storylines, but also some loose ends that make you think. It's a conclusion that divides opinions among fans.
One idea could be a crossover where the Naruto characters end up in the world of Heaven Lost Property. Naruto's skills in combat and his determination could create some really cool scenarios when faced with the angels and devils in that world.