How to Train Your Dragon (novel)/WMG

The book series will end with Hiccup uniting the Viking tribes
And he'll marry Camicazi to cement relations between the Bog Burglars and Hairy Hooligans. Fishlegs will wind up in Rome somehow and end up becoming an important political figure (again, somehow) who will put a stop to the Romans attacking Viking lands. (He's mentioned a few times how he'd like to go to Rome). If you read the epilogues, Hiccup is always talking about how all the tribes united in the end. It's been foreshadowed enough, especially with how well he and the son of the Meathead chief got on in the first book (compared to their fathers). The man who threw the sword in Hiccup's dream in How To Be A Pirate's epilogue was future Hiccup, and in the final book, he will do that.
 * Um...Rome never invaded viking lands. In fact, the Western Empire collapsed in the late fifth century. The viking age begins in the late eighth century. The city of Rome did exist during the viking era but it was just a backwater whose only claim to fame was the Pope and the ruins.

The tenth book will contain Hiccup going through extreme Break the Cutie.
If the previous books are anything to go by, the series is slowly turning Darker and Edgier as it goes on, but book nine still contained a surprising amount of death and darkness in it. The tenth book will complete Hiccup's journey to adulthood by bringing him close to breaking point and tearing away his former optimism. He'll get better, though. Hopefully.