What about a weekly challenge, where:
- you have to finish e.g. 8-10 fights, where each opponent is customized (no random like arena)
- you cant use some cards, because there are blocked/banned for the challenge
- you can pay gold to get a battle preview (like: New feature: Battle Preview)
- you get rewards at the end of the week for finishing the challenge
- you get special bonuses for the less attempts you needed
You are forced to change you build, coz some cards might be blocked.
You are forced to upgarde some cards you wouldnt upgarde otherwise to finish the challenge.