I think to have Black Manta as a sole villain (or even the main villain) would be a mistake. I think he should come back as a bigger threat this time, be dispatched and be saved for the third film.
In terms of who else I'd want (in no particular order):
- Siren, infiltrating Atlantis and posing as Mera, but perhaps make her a clone or something of Mera as opposed to a twin sister we've never heard anything about;
- The Dead King, leading an army of the undead to claim his trident and his throne back from Aquaman;
- Charybdis, as a gun-for-hire engaged by Black Manta who *somehow* develops superpowers and a monstrous appearance and causes Manta's plans to get out of control.
I'd be happy with any of them.