Concept idea 1.
This idea has no fandom and is an original idea, as such it can be put to almost any fandom providing you can shape your story around it.
Marco Loza is just an ordinary teenager (Spanish, tanned skin, brown hair and eyes) until the day he finds a mysterious tunnel in the park that hadn't been there the day before. Marco goes exploring in the tunnel and when he comes out he finds himself in another world, when he tries to go back he find the tunnel gone and himself trapped.
Marco is in an endless jungle with strange and deadly creatures and plants, the world he's on has three moons, longer days and nights, and no recognisable constellations.
By a stroke of luck Marco finds a large tree cage to take shelter in from all the carnivorous animals that want to eat him, but he eventually has to leave the protection in search of supplies. One day Marco comes across a animal tangled in vines and helpless, in sympathy he sets the animal free. Later on he is attacked by a pack of animals and is about to be killed and eaten when the animal he saved comes by and saves him. It is then that Marco learns that he can understand and talk to animals, the animal who saved him explains that it saved Marco because it was returning the favour for when he saved it, it then explains that other humans had come to this world but never survive for very long. Marco names the animal Chase and they form a mutual relationship.
Marco lives in his tree cage and searches for supplies and other things with the help of Chase. Humans aren't the only things that end up in the world, packages of food and clothes, cars, etc also end up there, which Marco scavenges.
One day Marco and Chase hear a scream and in following it they come across a girl being attacked, they rescue her and take her back to the tree cage. The girl's name is Alice and she says that she had just woken up in the world, later on they also find out that she has healing powers that she didn't have before.
As years go by more and more people end up in the Wild world in various ways and Marco rescues who he can and brings them to the tree cage to be safe, they end up creating a village in the treetops with the tree cage as the base. The village finds out a few things over the years; everyone who comes is under the age of 16, they can't age past the age of 16, and each gains a power or special ability that they didn't have on Earth. Marco is the leader of the village as he's the one who made it possible for them to live in relative safety, and he helped train some of the animals for the villagers to use to go out into the jungle.
Years later from when Marco first arrived and created his village he finds out that a large group of savage animals that are extremely hard to kill and defend against are moving closer and closer to the tree cage, meaning it would be even more dangerous to leave the tree cage in search of food or other supplies and that their village could even be attacked and destroyed. Marco concludes that they must find a new location for their village, so he and some others set out to find a new location.
After a few weeks Marco, and those that survived the exploration with him, find an island at sea which is safe enough to set their village at, the island doesn't bare much good food so the only animals that live on it aren't very dangerous to them. Marco sends world back to the village and the villagers leave the tree cage for the island before the savage animals reach them. Living on the island is much safer than the mainland but the villagers still have to go there to get food and supplies since the island doesn't provide for them very well.