I've just looked at Wiki's list of serials by year, and I see that 1935-36 are the years that the serials start getting more extravagant with their use of SF-tropes. Before that the early years of sound look a lot like the silents in terms of content-- westerns, jungle adventure (with or w/o Tarzan), aviators-- though I confess I'm no expert on the silent chapterplays.
The most "far-out" serial I see in the early 30s is probably Bela Lugosi's RETURN OF CHANDU, and it's also one of the few serials based in a recurring character from comic strips, radio, or prose (usually, but not always, from the pulps). Not counting Tarzan since he'd been in films since 1918.
Then in '35 you get SF-blends with a western milieu (PHANTOM EMPIRE) and a jungle milieu (THE LOST CITY), and '36 brings us the first FLASH GORDON, a second comic-strip adaptation (ACE DRUMMOND), two more SF-concepts (DARKEST AFRICA, UNDERSEA KINGDOM), and a couple more mundane serials with rough "super-villains" (CLUTCHING HAND, SHADOW OF CHINATOWN).
After that we get loads and loads of adaptations from other media, as well as original "superhero" films like MASKED MARVEL.
So-- what took the serial-makers so long to exploit properties from other media? Usually filmmakers interested in recurring characters are trying to seize the moment that they're still popular. I guess FLASH had pretty quick turnover, but we don't get BUCK ROGERS, whose strip probably caught fire in the early 30s, until 1939, or any of the pulp-heroes popular in the same early 30s period either (SPIDER'S WEB is 1938, SHADOW is 1940).
So serials seem to have been kind of timid about fantastic elements in the early 30s, when most of the studios were trying to ride the horror craze, and turning out many of the great classics. Did something happen to convince the money-men that a serial like FLASH GORDON could make-a da moolah, or that SF-tropes in general could be a draw, at least for juvenile audiences?