One of the most prevalent themes that "bottom-up" (mechanics-first, rather than flavor-first) blocks of Magic have been based on is multicolor matters. The reasons for this are clear: Players like multicolor, it has a lot of variations (allied pairs, enemy pairs, allied triple shards, enemy triple wedges), and it's fairly easy to design around. We've seen it in Invasion, Ravnica, Shards of Alara, and Return to Ravnica blocks, and thus far only wedges have yet to get the spotlight.
However, there's been more and more multicolor in recent sets, making it feel less special. Mark Rosewater fielded a question to this effect recently; sirzapdos pointed out on Rosewater's Tumblr blog, "Zendikar was the last "expert" expansion to not have a multicoloured card. Will we ever see a block expansion without a multicolour card again?" Rosewater replied:
We’re much more willing these days to have a little bit of multicolor in a set that I could imagine it might be a while until there’s a set with zero multicolor cards. I imagine it will happen when there’s a reason not have to have any in a set.
The most obvious reason for a set or a block to lack multicolor cards would be a monocolor theme, something we have yet to see because monocolor is less easy to rally behind than multicolor is (after all, most cards, period, have only one color) and it's harder to design a diverse environment when everything boils down to white, blue, black, red, or green.dek. There are a number of ways to accomplish a diverse-feeling monocolor environment, however: Having factions within single colors Fallen Empires-style (aggro-white versus control-white, for example, red burn versus red creature swarms, or suicide black versus black mill), judicious use of either multicolor or monocolor hybrid cards (the former to force a choice between which color to play them as, perhaps with "all-in" pay-one-color-of-mana-only effects or different scaling abilities depending on how much of each color of mana is spent), or simply pairing it with an artifact or colorless (Eldrazi?) subtheme.