For me it's in between Garth and Andy Curry. If Arthur dies I think Garth should be the next ruler of Atlantis even if he's not the blood heir (he is still considered apart of the Royal Family and Prince of Atlantis in canon) because he has more experience and just because my boy Tempest deserves more attention and maybe Mera and Arthur declaring him their heir could be a way to make it up to him after Arthur tried to kill him (not that being king is the best thing ever). Or Andrina decides she doesn't want to be queen and wants to focus solely on being Aqualass (later Aquawoman) with Jackson and gives it to Garth anyway. Maybe Princess Andy can be queen after him. DC can just make it subtle, like the way Nubia is queen instead of Diana. And Jackson should get to be Aquaman but Garth gets to be King.