I think their story will end with them getting their mojo back after a lack of sexual attraction, but we won't feel good about it as the audience, because it means they will wind up taking a cue from Cam and Daphne's dynamic and "play games" with each other. Thematically, that makes sense to me and follows Season 1 ending with the tragic inevitability of the new wife staying with her husband for the money.

Ethan will wind up killing Cam in the ocean after he tells Ethan that him and Harper were hooking up, until he ruined it and came back to the room. We still won't get it fully confirmed from Harper whether or not it actually happened though. Harper will figure out that Ethan did something bad to Cam, but it is basically unspoken mutually assured destruction for the both of them at that point to go through the whole truth of the situation. That, in turn will ramp up their attraction for each other in a slightly sad, disturbing way.

The punchline of it all will be that Cam's death will get lumped together with whatever deadly end the Quentin storyline takes, noone will suspect Ethan, and Daphne won't even care that Cam is out of the picture because she can go be with her trainer full-time now. Especially if it is revealed that Cam didn't have all the money he says he did.