A couple of blocks ago, the Gothic themes of Innistrad block were demonstrated partly through its tribal subtheme. The Humans, in green and white, banded together against the clannish monsters (Spirits in white-blue, Zombies in blue-black, Vampires in black-red, and Werewolves in red-green). However, as Mark Rosewater has revealed, there was originally planned to be a fifth monster tribe, one that may well be seen when Magic returns to Innistrad.
When mtxiii asked Rosewater on his Tumblr blog, "Will we ever get more Scarecrows?" he answered:
Yes, I believe you will. Scarecrows should show up on planes where they make sense.
Though Shadowmoor is the only plane so far to have a major Scarecrow population, Innistrad definitely springs to mind as a sense-making place.
This quickly prompted a follow-up question from seregthol, who asked, "But if scarecrows show up on planes where they make sense, why was there only one scarecrow in Innistrad? (and not even a very interesting one)" In his answer, Rosewater revealed something:
We originally had a scarecrow theme but it pulled focus from the rest of the design. We decided we wanted to focus on the five ally tribal races (vampires, zombies, werewolves, spirits & humans).
The discussion continued when nottinghillnapoleon asked, "Hi Mark, You mentioned in another question that Innistrad had Scarecrow theme originally but it was taken out to refocus attention on the 5 tribes; were scarecrows going to be the colorless tribe?" Mark answered:
Yes, I had a mini scarecrow theme early on because they felt like they fit horror, but cards and themes have to serve the set and the scarecrow mini theme was messing up a lot of the balance of the design, so it had to go.
Of course, since Innistrad block was one of the most well-received blocks of the past few years, it's only natural that R&D may want to revisit the setting. When they do, they'll definitely make the monsters a threat once again, since Avacyn Restored was not well-liked as a disconnected finale to the block - after all, Sorin created Avacyn as a desperation move, and even she and her Angels were barely enough to keep the people of Innistrad safe. To avoid totally retreading all the original Innistrad themes, new tribes will definitely be introduced - and Scarecrows are at the top of the list of those that will.