Honestly, if the rumours are true and an Aquaman film is announced at SDCC then a lot needs to be taken into consideration. The first thing would be that, as evident in The Others and Justice League, Aquaman is at his best when he is in a team. Although I would prefer a Team 7 movie before Justice League, that probably won't happen any time soon. So, honestly, in my opinion, if Aquaman is used as the first non-Superman film in the DCCU, then it has to be a team-up film. I would personally say an adaptation of The Others and Aquaman 0, but there are a few other choices that work out well as well.
While normally I say a superhero movie needs to be about itself, a reason why the new Fantastic Four is going to be so hard to adapt, so that the audience learns to care about the heroes, not the world they are in. Aquaman, cannot work this way. He is a hero who two decades ago was badass, a decade later B-List and now the brunt of Family Guy jokes. A film that builds him up needs to have other star power in it. Here are the options:
1) Wonder Woman - Aquaman is deeply rooted in Greek Mythology, as is Wonder Woman. An easy way to do this would be a war between Amazon and Atlantis that is being manipulated on both sides by a common enemy. On Atlantis - Ares is manipulating Orm. On Amazon - Ares is manipulating Hippolyta.
2) Shazam - No, not Captain Marvel, adding Marvel now would be a huge mistake. If Shazam is brought into consideration, then Aquaman would be the best way to jumpstart it. Have Billy Batson be an orphan who helps out Arthur Curry and gets him to embrace his destiny. It would also add a huge needed touch of lightness to DC Films.
On to Superman, though, obviously we will be getting Man of Steel II, hopefully called Last Son or Man of Tomorrow, around 2015 or 2016. So expect Aquaman a year or two afterwards. The best way to do this, as I believe I've said before is Lex Luthor. Don't imprison him at the end, though, and keep him active, with the people split between Superman and Luthor. Set up Man of Steel III with Luthor secretly funding Bruno Manheimm of the Intergang, use this as a way to put Darkseid in the franchise. Let's say he used Manheimm as a host to enter Earth. Superman banishes Darkseid, but only temporarily, lets say.
Since the New Gods are deeply rooted in Wonder Woman, this could set her up. If she's already set up in Aquaman, though, then in the sequel to that film we could see Steppenwolf or DeSaad. It's all to set up Justice League in the end, though.