I think moving Aquaman to December is actually a vote of confidence in the movie.
December is a month for HUGE sales in the movie industry. You don't put a movie you're afraid will be a dud in a timeslot like that one. I think WB are convinced already that they've got a winner on their hands and they're capitalizing on the opening Cameron created by pushing back the Avatar sequel to put it in the best possible position to really rake in the money.
As for the second movie? Sirens would definitely be a likely scenario. It's already been mentioned that the movie has been fast-tracked, so if any movie is in position to be ready for 2018, that's the most likely candidate. I suppose Justice League Dark is a contender too. That's another movie where they already had at least some of the work done already, since they're still using at least the framework of Del Toro's vision for the movie. At last report, anyway.
And yes, I agree with Reservoir Dog. If WB just can't get a second movie together for 2018, then that's fine. I don't want a hastily slapped-together movie just so we can have two movies that year. Quality always trumps quantity when it comes to movies.