I can't tell you how much teeth enamel I've lost reading Aquaman and seeing him the top 10 sales estimates, beating X-Men and Avengers for crying out, month after month. I'm blaming the announcement of the big JL event, THRONE OF ATLANTIS for the destruction of my crown, which I now have to replace. Yes. I am GREEN with envy.
I believe most of that success is due to Geoff Johns, big cheese in DC editorial. He loves the character and wanted him to do great and systematically set about making it so. This will never happen to Namor under the current regime. There's zero editorial love for Namor, especially when you have Marvel's EiC admitting he hates Namor and is hoping he gets killed off. Marvel editorial loves Joe Q's Marvel Knights characters -- and the movie characters. Pepper is right. Our best hope is a Namor movie, but now that Universal appears to have lost the rights, I don't see much interest from Marvel Studios.
And yes, I'd love to see more of Namor's underwater world and life fleshed out. Ralph Macchio once said he'd love to edit a Namor book with a fantasy / Lord of the Rings sensibility. That would be the coolest! With Lemuria and Attuma and other Atlantean colonies, you could even do a Game of Thrones riff. We've seen what a bunch of backstabbers there are at the Atlantean court with Krang and Byrrah and Farah, and even that less than believable secret eugenicist society, that could all be put to great Machiavellian use. There's been Lovecraftian elements in place since Roy Thomas' run with the Serpent Crown, and Stuart Moore touched up on it with the Aqueos living in R'lyeh, that could play into the magical threat, like the Others.
Sadly, no one at Marvel seems to want to do epic political fantasy like that.
I've tried for something like that in CBR's fantasy editor's league, Age of Marvels. I even drafted George RR Martin and Simone Bianchi as my creators for the book. But I'm no GRRM, and we only write up solicits, not stories. But hey, if you want you can come by and vote for it at CBR Age of Marvel.