I would like to see restricted format arenas in addition to current arena. Think like the different MTG formats. For example:
Day1 arena: can only use characters that were available when the game launched.
Follower arena: can only use characters without leader abilities.
Rotating limited: every week a random selection of 30 or so characters are selected and only those can be used.
Each of these could have a limited crystal payout (like 100 for #1) or maybe have a cost of entry (50 crystals to join each day) with top 100 getting at least 50 crystals and higher ranks getting better payouts.
Just a thought on how it could be spiced up a bit.