In 'gravemind oc fanfiction', it often focuses on an original character's journey in the world associated with the Gravemind. The story could explore different aspects. Maybe the OC is a human or an alien who accidentally stumbles upon the Gravemind's domain. They then have to navigate the dangerous and mysterious environment, deal with the Gravemind's influence, and possibly find a way to either escape or change the situation for the better. This can involve a lot of world - building based on the existing Halo universe and character development for the OC.
One interesting concept could be the fusion of Naruto's world with the Gravemind's characteristics. Maybe the Gravemind's hive - mind thinking influencing the ninja clans in Naruto's world. For example, how the shinobi would adapt to a new form of communication similar to the Gravemind's.
First, research about the Gravemind. Know its characteristics, powers, and role in the Halo series. Then, create your original character with a clear backstory, personality, and goals. Start with an interesting hook, like the OC's first encounter with the Gravemind or a mystery related to it.
It blends the elements by combining the ninja culture from Naruto with the alien and often menacing nature of the Gravemind. For example, the Gravemind's way of spreading its influence could be compared to how a powerful ninja clan spreads its ideology in Naruto's world.
At the moment, there's no definite information on whether Berserk manga will continue. It might, but it could also remain unfinished. A lot depends on the circumstances and decisions of those involved in its production.