The Legend of Uzumaki Hiroto was a derivative work of Naruto, which told the story of Uzumaki Hiroto's growth and the stories of other characters. Uzumaki Boto Legend was different from the original Naruto Naruto Shimbun Legend, so it was considered an independent manga.
If Merlin died in the fanfiction, it could trigger a series of tragic events. The balance of power could tip, and the villains might seize the opportunity to attack. It could also bring out hidden strengths in the remaining characters as they try to cope with the loss.
The Uzumaki manga is not overly long or short. It's just the right length to immerse you in its creepy world. The number of volumes and pages can change based on the format you come across.
Perhaps some of the snooty nobles who look down on Merlin and constantly try to get him into trouble. They could be making fun of his clumsy ways one too many times and he just snaps and hits them with a shoe. It would be a moment of satisfaction for the readers who don't like those arrogant characters.
First, give the OC a backstory that ties into the Uzumaki clan. For example, if they were exiled from the clan due to a misunderstanding. Also, give them some Uzumaki - like abilities like strong life force or sealing skills.
Well, he could start a family with Hinata and have lots of kids, passing on his Uzumaki bloodline. Then, he could encourage his friends who have some Uzumaki connection, like Karin, to also have families and pass on their Uzumaki genes. He could also set up a school or training center in the Uzumaki land, teaching the Uzumaki ways of life and ninjutsu, which would attract orphans or those who want to be part of the Uzumaki clan.