Large sets have 15 mythics. They are usually two monocolored ones of each color, and five of any combination. Most planeswalkers are not included in that monocolored count, and the rest is filled with artifacts, lands or random gold cards that don't adhere to the set structure.
Theros has two monocolored mythics per color, and five mythic gold cards. The last five mythics could have easily been the three planeswalkers plus two artifacts, lands or meaningful gold cards. Instead, Elspeth is counted in white, and we get the legendary Medomai, but also a Cerberus () and a random archon (), the only nonplaneswalker gold cards in addition the uncommon and rare cycle. I could have seen something like the Archon been designed as a rare, and being swapped to mythic, thus leaving no rare. But they did complete everything, so how the slots where chosen is not coincidental.
I'm guessing, then, that the next set will have 5 mythics in the following (missing) color combinations: , , , , in addition to the five expected monocolored ones. Some of these will be planeswalkers that were chosen as those colors because they don't have space to play around.
They clearly want a strict ten-guild God cycle, yet they unbalanced the colors in a very strange fashion in the first set without the need to, so I'm guessing the other Gods will just be rare.
And this is also a new excuse to think Ajani may appear as the first planeswalker.