If Jason Momoa could pick any two DC characters to appear in Aquaman 2 it would be Wonder Woman and The Flash. A sequel to Aquaman has not been green-lit officially. There are rumors though that a sequel is in development and given the critical and potential financial success of Aquaman, a followup seems like a certainty. The DCEU's effort to create a solo story for Arthur Curry looks like it's exactly what the troubled movie universe needed, and at the very least, it's better received than Justice League.
Aquaman is very much a standalone story. There are mentions to the events of Justice League but there are no cameos or storylines that hint at anything else going on in the DC Universe. Arthur Curry interacts with characters who are traditionally Aquaman figures. It's not too different from a Phase 1 MCU movie in that manner. Yet, if the DCEU does take more cues from the MCU, a sequel could (and probably should) see an appearance from at least one other Justice League hero.
In an interview with Collider, Momoa was asked if any DC hero could have made a cameo in the original Aquaman, even in the background, who would he have chosen. The actor didn't waste much time in answering The Flash, not only because he loves the character but actor Ezra Miller is "like family." Yet Momoa admits that Flash couldn't just be cameo. He would require a bigger role. This brought up the discussion of which DC heroes should get meatier presence in a potential Aquaman 2. Momoa answered:
I think Wonder Woman would be pretty rad too. Flash and Wonder Woman would be my first two. I don't want to pick one, so I'll pick two.
Before fans get too excited about Momoa's suggestions, the actor probably doesn't have much control on which, if any, characters will be in the still hypothetical Aquaman sequel. Unlike his DCEU co-star Ben Affleck, Momoa has only ever been attached to the movies as an actor. There's never even been a discussion of him being in a producer or director role. However, since Momoa's performance is continually one of the most praised elements of Aquaman, it's conceivable that he could get more creative control in Arthur Curry's cinematic future. This is pure speculation, of course, but it's not totally unreasonable for Momoa to get his wish and to have Wonder Woman and/or Flash appear in a sequel.
This is especially true concerning Gal Gadot's Princess of the Amazons. Outside of Aquaman, Wonder Woman is the DCEU's only other true critical and commercial success. If Aquaman does as well as suspected, the two heroes will be firmly established to be the stars of the cinematic universe. It would be perfectly natural for Warner Bros. to capitalize on that success and have the two team-up, outside of a Justice League sequel. A Flash role is a little bit more difficult to imagine as his solo movie has been delayed several times and his character doesn't mold as well into Aquaman's world, at least not as well as Wonder Woman.
Wonder Woman obviously can't survive underwater as long as Aquaman or any of his supporting characters. However with her background in Greek myth and being a member of a long-forgotten society, she does fit into the Arthurian world of Aquaman perfectly. There are obvious thematic parallels between Wonder Woman's Themyscira and Aquaman's Atlantis. There's also a lot more stories and examples of Wonder Woman and Aquaman teaming up in the comics then there is with The King of Atlantis and The Flash - even if some of those stories see them as rivals, not exactly allies.