Yeah, yeah! Remember all those awesome self-contained Venom miniseries?
I'm not sure I agree with you completely on the idea that cosmic novels work because they're finite. Lots of novels are part of lengthy epic series that never seem to end like the Wheel of Time series and Discworld and The Terry Brooks novels and the Dragonlance books, etc.
I suspect it might have to do with the art form's focus. Novels take you inside a character almost immediately if they're good. Comics tend to keep you in the exterior taking everything in visually. But then again, BONE worked and it was set in a fantasy land. I think I could have sustained interest in a bone comic that went on perpetually like the way say Scooby Doo or Superman do.
But yeah, Aqua man is defined too much by where he comes from and not enough by his character. I mean, Batman SHOULD be more boring that aquaman, if we go by what powers they have, but no one ever gets into Aquaman's training or skills or personality in any way that makes him stand out.
I know that Aqua hook got dissed up above, but I enjoyed that run because it gave Aquaman a distinct personality for the first time. He was a man struggling to be King and all the best parts of that comic came from that aspect. If you think about it, the character dynamics of an Aquaman comic shouldn't be any different from a Black Panther comic, should it? Isolated kingdom and culture, wary of outsiders but needing to go into the larger world to solve problems every now and again...