Seems to me the obvious choice for the sequel would be the reversing the threat; we had Atlantis preparing for war on the surface, now the surface is preparing for war on Atlantis.
Because of the events of the first film Atlantis is exposed and the US, China, Russia etc are terrified to learn they aren't the most powerful nations anymore.
Black Manta, having knowledge of Atlanteans and experience fighting them, gets the backing of governments. They give him money, special ops soldiers and perhaps some nuclear weapons.