One oft-repeated criticism of Theros block is that it failed to adequately deliver on part of its premise. Though it is a top-down Greek mythology block, which flavor it inarguably carries through, it is also touted as the "enchantment block," which only forms a part of its mechanical basis.
Still, that doesn't preclude a "true enchantment block" in the future, or even in the near future. When sarroth asked Mark Rosewater on his Tumblr blog, "Theros block seems cool. Bestow specifically does a great job of showing the gods' effects, and certainly justifies using enchantments. I think you were clear that Theros was not going to look or play like a bottom-up enchantment-block-like-Mirrodin-was-for-artifacts since it's a top-down Greek block already. To me it seems the dislike of Theros' use of enchantments is rooted in a worry that such a bottom-up set won't ever happen now. Surely that's still on the short list though, right?" Mark responded:
If enough players want something, we will eventually make it. Magic is a hungry monster. : )
"The enchantment block" - that is, in a bottom-up sense - is something players have been clamoring for since "theme blocks" became a real part of Magic design with Invasion.
That being said, even after Onslaught's "creature block" and Mirrodin's "artifact block" quickly followed, it took until Zendikar to get the bottom-up "land block" and an "instant/sorcery block" has yet to materialize. Permanents, however, are much easier to build a whole theme around than instants and sorceries are, and enchantments are essentially owed their due given all of the other primary permanent types (planeswalkers being temporarily disregarded because a "planeswalker block" would be impossible) have had a bottom-up block so far.
There is also the notion of interblock synergy, and while it's doubtful that Theros will have an enchantment block to share Standard with it, there may be room for another in the current seven-year plan.