The genesis of this set is my obvious enjoyment of off-color activated abilities. I have a few basic concepts so far:

• 5 Monocolor factions, each of which has a specific focus and employs each of the other colors in kicker costs, activated abilities, and other such uses of off-color mana
• There would be no true multicolor cards in the set
• Possible use of "twobrid" costs
• Possible focuses: White= Enchantments (overdone by me though), maybe tokens; Blue = Spells; Black = Graveyard; Red = Artifacts (inspired by C14 probably); Green = creatures, lands, or enchantments
• Returning mechanic ideas: Kicker, twobrid, flashback, evoke, unearth, Retrace
• Minor legendary theme?
• Each color has at least four creatures at common with an activated ability using another color of mana (I.e. White would have , , , and activated abilities)

Bit of flavor:

• Each faction has a monocolor at its color which is incfluenced by the philosophies and capabilities of each of the other colors.
• White needs a focus.
• Blue focuses on instants and sorceries.
• Black is all about recovering things from the graveyard. It uses the other colors to recover anything it wants.
• Red is all about creation and artistic expression (damn, I do this all the time!). Mechanically, it likes both artifacts and enchantments.
• Green has a large focus on creatures and tokens.