The ability to have teammates would add a lot

Some kind of a friends list would add a lot to the game.  I feel like thematically calling them teammates would work.

You've given the ability to create a game and give a code to only allow a specific person to join.  That's great.

I see the same people time and time again in matches.  I think it would be great to be able to send an invite to that person and be able to play the game with them to experiment with deck ideas with them.  This is a pretty normal feature set for games like this.

* Battles that do not have to risk cube gain/loss to foster experimentation. 
* The ability to limit card series/rarity in some way to level the playing field within a rule set so if I play with a new player we can limit the types of decks that can be used so I don't destroy a brand new player with Thanos and Galactus decks ... which would also increase the value of less compeditive cards for use in these more friendly matches because if we're both limited to series 1, 2 and 3 cards that will change many deck designs drastically.
* I don't know if this really needs to have full clan features with chat and all of that.
* I would want to be able to see how long ago a friend/teammate was last online to be able to clean up inactive players.  You would also want to know who was online at that moment in time.

Beyond the strategies and victories, four in a row is about creating cherished memories. It's a game that bonds friends and family, leaving you with stories to tell.