I've been wondering about this for a while. We know that the deck size has to be a multiple of 3 to allow for an equal number of each of the 3 Houses. But why not 30, 33, 39, or 42? Was the number 36 a result of lots of playtesting that people can shuffle their deck at least once in a game?
Recently, I realized that 36 is not only a multiple of 3, but also of 2 and 4. If FFG wanted to, they *could* make a Keyforge deck with just 2 Houses in it, or 4 Houses in it, and they wouldn't need to change the deck sizes. I'm not saying that this would be a good idea: 2 House decks would probably need to be full of sub-par cards, and 4 House decks would need a lot of Omni cards and really good cards to balance out going between 4 Houses. But whether or not you think it's a good idea or not, I like that they thought far enough ahead to make it possible.