You know what, screw it. I want to see Namor as the villain in the first movie. That's spoiled a bit because he's supposed to be in the Black Panther movie. But they could have one lead into the other if they want. Maybe set Namor up in BP2, and have him launch a full scale attack on the surface world in FF. Reed and Sue can be engaged, or close to it, but Namor still sparks up the sexual tension with Sue. Have Doom in the background as having negotiated a peace treaty to keep Latveria out of the line of fire. Maybe even whispering in Namor's ear to spur him on.
I think this works best. Namor and the Atlanteans provide an epic threat, and we haven't seen this before. At the end, Namor suffers sort of injury, and Reed designs a suit to save him. Resulting in an uneasy end of hostilities.