One idea could be that the dragons in HTTYD are attracted to the cold of Frozen's world and they fly there. The people of Arendelle are initially scared but then they learn about the dragons from the dragon riders. Another idea is that Elsa's ice powers somehow have an effect on the dragons, like making their fire breath turn into ice breath for a short time.
First, you need to understand the characters and worlds of both HTTYD and Frozen. Then, think of an interesting plot that can bring the two together, like maybe a dragon from HTTYD somehow ends up in the Frozen world. Next, start writing the story, focusing on how the characters from both universes interact with each other.
One idea could be to have Toothless meet Jack Frost. Maybe Jack could try to ride Toothless and they could have an adventure together exploring the skies and the wintery landscapes. It would be cool to see how their different powers interact, like Toothless' fire against Jack's ice.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
One idea could be that the dragon's fire power can be used to recharge the Transformers in some way. Another is that the Autobots could learn the concept of flying from the dragons and incorporate it into their own designs. Or perhaps the Decepticons try to capture the dragons to use their power for evil purposes, and the vikings and Autobots have to work together to stop them.
First, you need to study the characters and settings of both 'How to Train Your Dragon' (HTTYD) and 'Frozen' well. For example, understand Toothless's character traits in HTTYD and Elsa's in Frozen. Then, think of an interesting plot that can combine the two worlds. It could be about a quest where the characters from both universes have to team up to face a common threat. Also, pay attention to the writing style, keep it engaging and easy to follow.
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.
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.
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.
One idea could be that the characters from HTTYD find a strange portal that leads them to the Pokemon world. There, they have to learn how to catch and train Pokemon to find their way back home. Hiccup, being the curious and inventive type, might try to combine the technology of Berk with Pokemon training methods.
One idea could be that Danny's ghost powers somehow interact with the dragons in HTTYD. For example, his intangibility might make him able to pass through dragon fire without getting burned, and this could be a crucial skill when dealing with a new, more dangerous breed of dragons. Another idea is that Danny gets transported to the world of HTTYD and has to use his ghostly abilities to fit in and help the Vikings. Maybe he can use his flight ability to race against the dragons, which would be an exciting plot point.