Sideboard is kind of a big deal for deck planning, is it still up for debate or has there been a decision? *EDIT* I actually don't think it's a good idea anymore, as it could allow some pretty hard counters.
Also, if sideboards were in, could you change your mark between games too? I think this would be a bad idea, as it would allow decks to change quite wildly. But for the sake of argument: pends in a mono deck could allow off-element splashes by using the sideboard and a change of mark.
Spoiler for EXAMPLE:
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–3 Cockatrice, +3 Deflag, Change to Fire Mark
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