Say, one day a week, maybe it's thursdays, each arena gets a different bonus.
Dragonspire might give plus 5% pierce Celestia might give plus 20% critical Marleybone might give minus 10% resist Zafaria might give plus 20% stun resist etc.
it gives this to all players on both sides, and you are assigned your arena randomly.
On arena bonus day, when you are starting your match you see a notification that says "today's Arena bonus is XYZ"
