The theme of the set is card types. Each of the five frequent types apart from creature is associated with a primary and a secondary colour:
Artifacts are WHITE-blue. The plane of Sienira has a number of reclusive human and dwarven artificers who sit off in their towers called musearies, crafting artifacts and acquiring reclusive reputations.
Instants are BLUE-black. The crime in the towns and cities of Sienira is ruled by several criminal families of rogues who are frequently making, discovering and altering plots against each other.
Sorceries are BLACK-red. The most visible religion is the Zerian church, who worship the demon Eharot with elaborate and disturbing rituals. The clerics are often seen accompanied by creepy elementals, summoned by their assorted rites and pacts with the demon.
Lands are RED-green. The steppes, forests and wild spaces between towns are home to many fierce beasts and hermit shamans, who have their own culture. Establishing a mana bond to an area is staking a claim to it, and the local shamans will contest you for it, or defer to you if you have more power.
Enchantments are GREEN-white. The high society of Sienira are obsessive about enhancing their experience. They like to set up a villa or stately home in a natural setting, but then hire enchanters to layer magical effect upon magical effect to appease their sense of aesthetics, augmenting reality to reveal the way nature wants to be.
There's a lot more about each of these groups on [the set's website]. Creatures are not affiliated; each of the five factions is associated equally with creatures.
My design aims were to create a set as close as possible to the way Wizards would currently create one, with only a few exceptions:
It's a standalone set rather than the normal "first set of three" pattern.
The set isn't intended to slot into any specific Standard environment, so there's no cross-block synergy with blocks before or after it.
I deliberately allowed the complexity level to go a bit higher than recent expert-level sets.
But everything else is trying to follow current Wizards design philosophy as far as possible. So that means current colour pie, current rarity structure, following all the standards of set design such as colour balance and so on. It's intended to be playable in Limited, as well as playable in constructed decks.
The set has been drafted once, using PyDraft. It went well. See /Drafting for the details. Next up is probably building some proxy constructed decks, and playing them either at GamesEvening, or online with the PyDraft shuffler.