Well, you start with the obvious
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Dragons of Tarkir was a two-color ally set
Players should gather before playing and talk about their decks, what they do, what things theyd rather not play against because they may make the game boring, and set up a series of core rules so that everyone gets to have fun
Dont forget that many cards with flashback can fit into this theme as well
But the last thing we want after the first release of a block is to have players focusing on the theme of next years block something they wouldnt be able to purchase for ten months. If I were a more snarky writer, this is where I might point out that we now get teasers for sets many, many months in advance, which in essence is WotC doing exactly what Rosewater is describing here as the last thing we want. But Im not snarky, Im a cool and wholesome guy