This time WotC bets for quality before quantity. Do you remember the number of prestige classes in 3.5? There were literally hundreds, and then the fall in the oblivion. And other reason is the 3PPs are publishing their own "crunch". But WotC can't bet too much only in the "fluff", the lore or background, because not only there are other publishers, but also in internet age we can read the fandom wikis about novels, comics, teleseries, movies, videogames, etc.
* If there is some possible project about the elementals, then these could be linked with the cult of
Tharizdun, the elder elemental eye. Maybe this is the main antagonist against the order of cronomancers as one of the main menaces of the multiverse. And the troubles are worse if there are agents working for the cult of Tharizdun from different alternate timelines fighting each other because theirs is the "sacred timeline". Yes, the idea of the "multiverse war" is not too original.