Here are a few features I would love:

1) Adding other sites to get prices from (not sure if it is feasible to get prices from SCG, CFB, etc) but it would be really convenient for me since people locally trade with StarCity prices

2) Hiding inventory would be really nice for me (as there are decks that I would like to make and cards that I own that I do not want people to know that I have) If a feature like that were added, I think that it would be a lot better for people who do not want to trade out of their inventory or have their friends see their super secret new deck

3) Entering cards by set would be nice. While importing spreadsheets is very helpful, when keeping track of my collection I need to use the TCGplayer collection tracker, since entering cards is a lot easier.

You can set decks to hidden and/or private that can only be accessed with a password. The only difference with this and hiding items on your inventory is that cards that are listed in the Decklist but you dont list in your Inventory will be marked Red.