gaunt atlas
I have been using this feature since day one, and even if the arena has been online for just three days, cards are a big issue.
At first, you will unlock new ones, mainly commons. But after some packs opened, you'll start to get duplicates. And there's no use for them, as you can't equip the same card two times.
My suggestion is to make something with those duplicates. Maybe a fusion with X number of the same card, where you spend gold and have a chance to get a new, rarer card, or get nothing; or even make the old cards (commons) directly superior rarity, boosting their stats with the same method.
worldly bison
little prawn