With Theros being released in a little over a month, we still know very little about the upcoming fall set. Though we do know a few things about the flavor of the setting, only seven cards have been spoiled so far, and we know only two of the five keywords. However, there will be some major previews at PAX Prime on August 30, and according to Mark Rosewater's Tumblr blog, preview articles start on September 2. The man himself has been dropping a few subtle hints about the set on Tumblr, because of the many fans asking about it (he has been waiting, as it were, for quite a while to safely and non-spoilerifically talk about it).
On the topic of worldbuilding, when user godavarism asked, "Did Theros's enchantment theme come from a perspective of "we want to do an enchantment block, and Theros seems like a good place," or did it come from "we want to do Theros, and it really wants to be an enchantment block"?" he responded:
As my column of the topic will explain, they kind of came hand in hand.
Enchantments don't necessarily jump out as the sort of thing in Magic that'd be associated with a world inspired by ancient Greece, at least not intuitively, but it'll be intriguing to see how this came about.
R&D has also discovered some interesting ways to build flavor in recent years, one of which is "trinket text," or abilities that are only marginally useful but are just too elegant, strange, or fitting to pass up. When cairnwanderer asked him, "I've long been a huge fan of 'Trinket Text', such as Baneslayer Angel's Protection from Demons and Dragons. While I understand why it isn't used in overabundance, I can't stress how much I adore it. With Theros being a largely 'top-down design', flavor-focused block, can we look forward to more 'trinket lines' than normal, or would you say that it has a rather normal amount of them?" the response was:
Theros has some trinket text you should enjoy.
This, of course, doesn't tell us much, but perhaps one of the hero creature cards will be Hercules-inspired and have protection from Hydras and from Cats, to reference both the iconic Baneslayer and the two most famous Labors of Hercules.
Another part of Theros that the rumor community is hotly anticipating is the trailer, which it turns out Mark is too. When suliscrew asked "When does the trailer for Theros get uploaded?" he replied:
I don’t know when. I can say I’ve seen it and it is probably the best one we’ve ever done. The creative team kicked some butt on it.
The set trailers have been getting more and more elaborate since they became a regular feature with Scars of Mirrodin. Perhaps we'll get even more advanced animation - though perhaps still in the motion comic style - as well as multiple narrators.
Finally, he let slip a little something about the keywords of Theros. When reatlvl99 asked, "Which new mechanic in Theros eats up the most complexity points at common?" he said:
You haven’t seen it yet.
At first blush this seems to indicate the god mechanic, which might not be appropriate for common at all. Then again, the Planeswalker's Guide to Theros, Part 1 said that lesser gods and servants of the gods have the same connection to Nyx that they do, so the same ability might show up for them in a lesser form.