Aquaman ended with Arthur Curry defeating his half-brother Orm and ascending to the Atlantean throne with Mera by his side.
One would imagine that this is what Aquaman 2 will be about: Shin and Manta using whatever forces and resources at their disposal to infiltrate Atlantis and cause trouble for the King of the Seven Seas.
Shin asked for Black Manta’s help to find the underwater nation, and the mercenary agreed in exchange for Shin’s helping seeking revenge on Arthur.
The underwater kingdoms were at peace again, but that can only last for so long in a comic book media universe.