Just a suggestion for being able to get cards you really need. I've gotten enough cards for GDI that I can get all of my epic units to level 6 or 7, whereas most of my epic Nod units are stuck at level 5 or not even unlocked. Not only that, but I'm halfway to level 13 and I only just got the Nod artillery card to unlock the unit because it was in the store. We have a mechanism to give cards to allies, but we can't do that with epic cards. How about a way to trade in your excess cards for a card of your choice, or even just a random one, of the same type of cards you traded in.
You'll see similar mechanics in many other games. For example, in CS:GO you could trade X amount of weapon skins for one higher tier weapon skin. This should come at a high cost though, maybe 3 epic cards for 1 epic card, 6 rare cards for 1 rare card, and 15 common cards for 1 common card.
I already have multiple builds that I find fun to play, but I can't play them competitively because the cards just aren't dropping for me to level up the units. With the matchmaking I get level 5 artillery and cyborgs just can't compete with level 7 or 8 units.