I know this is low priority right now, but just thought of it, so figured i'd post
1) There needs to be some form of calculator. Whether this is just a plain old (4 function) calculator added to the deck creation page, or something very different (for example marking certain cards as "first turn" cards, and it telling how much your first turn will cost)
2) It'd be cool to mark your cards "first turn," "in case of emergency," etc
3) Notes for each deck: Just like when you make a powerpoint presentation, there is a section for notes on each slide, add notes to each deck. This (and #2) will really help people who don't use a deck for a long time, or have many decks, or have bad memory like myself
4) For those who make map-specific decks, we need to be able to mark which map(s) the deck works well for, and have benefits from that while making a deck. For example: See the map while making the deck, tell the CP limit, starting gold, etc.