1-The possibility of adding a small note to each card (if storing such data is a concern, limit it to a single sentence - 20 words or so). Benefits: other people seeing your deck can understand why a certain card is there or which cards to exchange from the sideboard/deck. People returning years later to Magic/moxfield will be able to remember their choices and reasoning at the time.
2-Add the grid we use when playtesting (just the grid) as a small window over the page (similar to what youtube does to miniplayer - but bigger). User can copy the cards from the webpage into this floating grid and move the cards around. Benefits: visual aid when seeing (for example) all fog cards I have in my deck. I can see them in this grid clearly and better evaluate them (their mana costs, colors, how many I have in the deck etc). This grid is just for moving and seeing the cards I paste there, other features from the playtest page are not needed. It may also rest at the right part of the page -with a more vertical design- but that would likely require the cards pasted there to be scaled down (still works, but not as good because Magic players love looking at the cards and most of us don’t have good eyes anymore).
I have thought about these at least 100 times while creating decks. Much love to all.