and adding a card to the game that could be over exploited in arena is not something to think about?
The problem is that The new shard isnt what ends up being the problem card. The problem card ends up being Black Hole.
You dont need a thousand copies of the shard. all you need is a ton of copies of black hole. Sure the shard opens up this problem, but the only reason the shard is focused on is because it is coming second. If The Shard already existed and black hole was being introduced, people would be complaining about how OP black hole is going to be.
And no, it being exploitable in the arena isnt something to think about. If the card is already balanced, then it wont end up being a problem in the arena.