Whether these go under premium or not, I think these are all good features that should be added to the site. If all were added I would surely buy premium as the "Edition Checklist" thing is not enough to draw me to pay. I am coming from the view of someone using this site to catalog my collection for insurance purposes as well as simply knowing which cards I have, which I don't, and where they are. If many of these features are streamlined, added, and could be automated by a POS I could even see stores using this to inventory their collection. Ranked based on rough levels of importance:
1) Ability to one-click add 1x-4x of each common and uncommon from a set, 1x-4x of each rare, and 1x-4x of each mythic. My current strategy has been to buy a playset of each common/uncommon on set release so I never have to worry about it. This would make inputting to deckbox much easier and let me really only focus on sorting the rares.
2) Ability to one-click create an excel or google sheets spreadsheet of your inventory, tradelist, specific decklists, etc. This would be huge to be able to auto-generate a spread sheet with prices and card names/quantities instantly that I can send to my insurance agent if/when big price updates or changes happen. Could also be used by people looking for specific cards to print their wishlist and stick it in the front of their trade binder.
3) Better inventory search/add features. It would be nice to use the "deck" feature to help identify where on my shelf my cards are. I sort my commons and uncommons by set, but have separate boxes for rares, foils, and foil lands. Search options that let you filter your inventory by things like rarity, set, foil-ness, language, etc. then one-click add those cards to a deck would be huge. On the inventory page it should also be able to say which deck you have the card in when you hover over the "total number in decks" column. So if I have 6 Swords to Plowshares, one is in my EDH deck, 4 in my legacy deck, and 1 is in my revised box I should be able to hover and see that info.
4) Similar to #1, for each boxed product (Commander, intro decks, ftv, etc.) there should be a "bought it" button that auto-adds every card in that product to your inventory in the language you specify.
Any other ideas welcome.