More event customization options would be amazing. For instance, the deck format editor while amazing, can still use some tweaks, such as, perhaps forcing a player or groups of players to use specific deck formats, or the addition of "Card X must be a part of the deck" as opposed to "Deck can only contain Card X". Among other tweaks to the deck format creator.
Furthermore, there should be a way to have an event do options like, 2v2 fighting (as mentioned earlier in this thread), however only in events. This could mean that players can choose their partners removing the chances that you are paired with someone that doesn't understand your language, strategies or the meta game.
Other small tweaks that would save my time greatly when trying to find cards in my collection would be to add the option of "View recently added cards" which shows cards added to a collection in the last 24-48 hours.
Another interesting way to improve the collection interface could be to add a self marking system, or a favourite system in one player's personal collection to Mark
cards and then view all the marked cards in their collection.
Finally, I'd like to have the ability to rearrange the game modes on the playing interface to suit my personal needs. I.e. no need to have certain rooms come out first if I don't use them. Or an option to view all available rooms without scrolling left or right.
However I'd stay clear of adding certain rooms as that separates the amount of players per room to an even further extent creating more difficulties to find opponents.