Purchase-able Unlimited Deck Cap
Deck capacity, the way it is (exponential cost for a small change), discourages collecting, and is quite strict if you are trying to have a large collection that has many commons and uncommons worth very little. Many have suggested increasing the deck cap x2 for example, but that is only a temporary fix that doesn't help everyone. An unlimited deck cap that could be purchased after hitting 750 deck cap or something (1.5k with supporter) for about 1000 bank would, in my opinion, be the the best way to go about this as it doesn't change the game at all for most people, only the people who want large collections and are willing to pay a lot to increase the deck cap. If this is implemented, anyone who has a higher deck cap than the 750/1500 should be refunded. This would also prevent deck cap ever becoming an issue if, for example, a new season came out in the future and would let people who have massive collections, such as every x GCR card be able to do so with less problems.
Credit to Fhaengshia for the original idea, stating "[NationStates] could just make it so you can buy unlimited deckspace for 1000 bank"
Prevent Bids and Asks Under Junk Value + Watchlist
Bids under junk value are quite annoying in my opinion. Usually people use them to put a 0.01 bid on an expensive card they want to watch, but there are some who bid under junk value maliciously to earn off of new/apathetic players to cards. Adding a watchlist would remove the need to place 0.01 bids to watch cards you want to buy, which leaves only the people trying to do the annoying thing. The watched items could appear in the Auctions tab when they have an active auction, but only to the nation watching them with at tag saying [WATCHING] similar to the [OWNED] tag. An additional page for the watched items could appear similar to the value deck page, potentially with a custom ranked order. Watched items would be visible on the card page similar to the collection tab and any nation could look at another nations watched items. Upon implementation, all bids under JV could be removed and converted to watchlist items. It would also be nice to add whole groups of items into the watchlist like "Unowned S2 Legendary cards" or something.
Better Filtering and Sorting
As it stands, you cannot filter by season and rarity at the same time. This is a slight annoyance. This should be made possible as well as the option to sort any page with a list of cards by the various columns such as bid, ask, value, copies, etc.
Proper Trading
Currently, the best way to trade cards with other nations is to gift to each other and hope they don't scam you. There should be a trade system that lets you offer to someone a trade of cards, with them being able to accept, decline, or send a counter offer. Players should be able to disable receiving trade requests and block trade requests from certain nations. Trades, if accepted, would be functionally the same as gifting. Both nations would need the deck space and pay the transfer fee. All trading would be is a more reliable and safe way for players to trade cards with each other.
Gift Receiving Options
An option to disable receiving gifts or being able to set a whitelist/blacklist of nations you can receive gifts from would be a nice feature to have for people who want to self collect or prevent someone from sending them anything that doesn't fit their collection for example.
Please let me know what you think of these ideas and if you have any criticisms or additional ideas for them.