1. I stand by my comments that Cthuhlu (or a copyrighted DC equivalent)is Aquaman's beer buddy/nemesis. it also explains the star spawn we saw in young justice in a child exchange program not unlike the new gods.
It's easy to punch the big bad, but having a rival lovecraftean kingdom under the sea that he has to politic, rather then war with doubles the potential. So instead of ruling the 7th season, it's more 3.5 with the star spawned underworld as a rogues gallery for contention.
2. Honestly, Aquaman's Trident needs to be on the same level as Thor's hammer in regard to its usefulness. I absolutely love his powerset in injustice: gods amongst us and the ability to stick a trident through a portal to attack from a bizarre angle just improved the character's combat set considerably.
3. Pacific Rim lays down a concept that has serious potential in an aquaman comics. Open a portal and have monsters flood in that only the king of Atlantis can stop.
4. Atlantis is known for it's technowizardry and while i'm not sure how much Arthur actually knows, he should know the basics and have the equivalent of a few "emergency items" when his fantastic physical skills fail him. Tsunami bombs anyone!
5. Setup a series of undersea bases that are top of the line and may one day be a threat to Arthur's rule, but they are settlers that really just want to live in peace. Take Sealab and play it straight. This is also a place where Black Mantra has diplomatic immunity and would cause an international event, if Arthur pulled out the big guns.
6. With the deadening of the seas, build new monsters. Take the nestine consciousness and base it on the Great Pacific garbage patch as it has been animated by microscopic creatures to become a threat all it's own. Mutate jellyfish as some sort of natural eldrich horror that is immortal, which may or may not have have help from Cthuhlu's forces.
7. The abyss is a cold dark forbidden place that is just perfect for more horror orientated stories and one theory that keeps being kicked around is that there is an entire subaquatic cave complex even below that. What's in the depths that even Atlantis fears to tread? Let the writer's decide. C
8. Are whales and dolphin's intelligent life? If so, they might not be fans. What if an experienced Orca learns technowizardry and thinks Atlantis has been sitting around while his people gets kidnapped by surface dwellers and raises an "Armageddon pod" that raids the seas for equipment and supplies to wage war against them. They are known as the seawolves of the sea for a reason and if they built "walkers" they might have a fighting chance.
There's a ton that can be done to make things interesting and just throwing this out changes could give a ton of stuff to work with.