Perhaps the vikings of Berk accidentally open a portal to Spyro's realm while exploring a new area. Spyro is intrigued by the vikings and their relationship with dragons. Meanwhile, Hiccup tries to understand Spyro's abilities and how they can be used to improve Berk's defenses. They could team up to rescue some baby dragons that were captured by an evil force from both their worlds.
In this crossover fanfiction, Spyro could get lost in Berk. He's confused by the cold climate and the different type of dragons there. Toothless takes an interest in Spyro and shows him around. As they get to know each other, they discover that there are ancient prophecies in both their worlds that foretell of their meeting and that they are meant to find a hidden power source that can save both their realms from destruction.
Another plot could involve Spyro and Link from The Legend of Zelda. Spyro could end up in Hyrule. Link, being the hero, initially is cautious of Spyro. But when they realize they both have a common enemy - Ganon who wants to use Spyro's powers for his own dark purposes, they join forces. Spyro can help Link reach places he couldn't before with his flying, and Link can use his swordsmanship and knowledge of Hyrule to guide Spyro.
A crossover with 'Alice in Wonderland' could also be cool. Spyro finds himself in Wonderland and has to navigate through the strange and magical places. He could team up with Alice to defeat the Queen of Hearts who has somehow gotten her hands on a powerful magic item from Spyro's world and is using it to cause chaos in Wonderland.
Well, a great plot for a Spyro and Fairy Tail crossover fanfic could involve Spyro's dragon powers being mistaken for a new type of lost magic in the Fairy Tail world. The Fairy Tail guild members might be intrigued and try to recruit Spyro. Meanwhile, Spyro is just trying to find his way back to his own realm. He could end up sharing some of his dragon skills with the mages, like his fire - breathing ability, which could be enhanced by Fairy Tail's magic, creating a whole new kind of power.
One idea could be that the two Spyros, from different dimensions perhaps, have to team up to defeat a new powerful villain that neither could face alone. For example, a dark sorcerer who is draining the magic from all the dragon realms. Another could be that they initially mistake each other for an enemy and engage in a friendly sparring match before realizing they are on the same side. Then they combine their unique skills, like one Spyro's amazing flying maneuvers and the other's powerful fire breath, to solve a mystery in the dragon world, like the disappearance of an ancient dragon artifact.
A cool concept would be a hybrid character. Imagine a Spartan who has been genetically altered with some dragon DNA. This character could have the strength and combat skills of a Spartan along with the ability to breathe fire or fly like a dragon. Or perhaps the Covenant could be interested in the dragons in the HTTYD world because they see them as a new form of powerful weaponry to add to their arsenal.
In a rotg and httyd crossover fanfiction, perhaps the Guardians could come to Berk to protect it from a new threat. Pitch Black might team up with some dragon hunters, and the httyd characters along with the Guardians would have to work together. Hiccup could learn some new skills from the Guardians, like how to be more stealthy from Jack Frost, and the Guardians could be in awe of the dragons' intelligence and strength.
Another interesting concept is a villain crossover. Mother Gothel could team up with a dragon hunter from HTTYD. They could both have their own motives, like Mother Gothel wanting to use the dragons to keep Rapunzel trapped forever, and the dragon hunter just wanting to capture the dragons for profit. Hiccup and Rapunzel would then have to join forces to stop them. This would create a lot of action - filled scenes and also some interesting character interactions as they try to outwit the villains.
One idea could be a crossover with another fantasy world like 'The Elder Scrolls'. Spyro could find himself in Skyrim, facing dragons of a different kind. Another could be a crossover with 'Harry Potter', where Spyro enters the wizarding world and has to deal with magic there. Maybe he helps Hogwarts students in a dragon - related task.
One idea could be a human from our world getting transported into the Spyro universe. They could start off as a confused outsider but then gradually learn the ways of the Spyro world, befriending the dragons and other creatures. They might even discover a hidden power within themselves that helps in a great battle against the forces of evil in the Spyro realm.