Here are the rules for a variant tournament format. I'll explain my thinking at the end.
Deckbuilding
Standard deckbuilding rules apply: decks must have between 50 and 100 cards. A deck cannot include more than 3 of the same card, by name. Any deck may include Neutral cards. A deck may include Order cards or Destruction cards but not both.
In addition, a deck cannot include more of the same card than would exist in a card set comprised of a single Core Set and one of each expansion set. For example, a deck could include 3 copies of Innovation, since there is 1 in the Core Set and 2 in Assault on Ulthuan, but would currently only be allowed 1 copy of Troll Vomit since there is only 1 in the Core Set.
Banned cards
No cards are banned in Single Set v1.0.
Sideboards
Sideboards are not allowed in Single Set v1.0.
Tournament rules
Tournament rules for Single Set v1.0 are the same as the official Fantasy Flight Game rules.
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Alright, now for an explanation. It's been my experience that a fair number of new players coming into the game are drawn in by the lower cost of ownership to play competitively. They are not happy when you explain that competitive decks often require multiples of the core set and expansions. Because of this, I think that a substantial part of the player base effectively already plays the format I'm describing - they just want to have a copy of every card, but don't feel like spending the money for a full playset - so this format will help to bring the hardcore and casual players closer together. Casual players typically do not use sideboards and neither do the official rules so I decided to make that part of the format too.
As a bonus, I suspect that a format where Skaven are significantly weaker (but still exist as a force to be reckoned with) and the Bolt Thrower archetype basically doesn't exist will make for a significantly more interesting metagame. It may be eventually necessary to ban cards (e.g. Warpstone Excavation) but until this format is explored in depth I don't think it's necessary from the start.
Feedback is most welcome!