I was thinking it'd be really cool if there was a way to keep track of sideboard plans for different matchups. Currently, you could just make different decks that use the same 75, but if there was a way to create "sub-decks" of a maindeck that allowed you to see how you'd sideboard, it'd be a little easier.

Either a drag-and-drop type interface or just changing quantities as you do for current deckbuilding would work.

Obviously not a huge priority, I'm sure, but I figured it couldn't hurt to post the idea.

This is actually related to a feature I'd like to see: changelogs and deck histories. Instead of individually adding and subtracting and adjusting the numbers of cards, I'd like to just enter something like:

-3 Geist of Saint Trat
+3 True-Name Nemesis

and have the deck auto-adjust. A sideboard guide would be stored in a similar format.