The idea of iconic creatures - the typical types for big splashy rares of each color - is a familiar Magic concept, easy to grasp because for the most part the said icons keep entirely to their own assigned colors. In recent sets, with few exceptions, white means Angels, blue Sphinxes, black Demons, red Dragons, and green Hydras. There is, however, one wrinkle in this tapestry - the case of the Dragons and certain very high-profile horizontal (five-card) cycles.
You see, Dragons are far and away the most recognized and varied creatures on this list in fantasy in general, so there are tricolor legendary Dragon cycles in Invasion and Planar Chaos, a monocolor one in Champions of Kamigawa, and one of nonlegendaries that cost 4CC in Mirage. This is not even to mention the Legends Elder Dragons that started it all. Almost all of these cycles were very positively received, and the Elder Dragons even resulted in the now-extremely popular Elder Dragon Highlander - aka EDH, aka Commander - format being built around them. So not only is there a lot of precedent for Dragons in every color, there is also a lot of demand. But why should they have all the fun, and the other four iconic types be fenced in by their own colors?
When obazervi asked Mark Rosewater on his Tumblr blog, "So we've seen full cycles of dragons; any chance we'll see any cycles of angels, demons, sphinxes, or hydras?" Mark put the question up to a poll, which got enthusiastic support for all of the above.
The question's asker replied "I’d love to see a cycle of demons that highlight the unpleasant sides of each color," while dmisetheharbinger wrote "The easiest two to execute would be demons and angels, however, outta quirkiness I would want to see sphinxes done in a full color cycle as like a world council :) the green one would be weirdly awesome," and dts87 said, "Just combine the iconics so we get an Angel Demon, Sphinx Hydra, Dragon Angel, etc. I leave it to creative to figure it out. : )"
I, personally, am pulling for Hydras with heads representing different elements. What about your choices for cycled iconics?