I'm thinking you could go even further with this idea and have specially tailored conditions that come with bonuses. You would have a cool way to experience PVP tournament-style deck-building because of all the limits. The oracle could generate 3 additional conditions that affect how you build your deck and it could include things like:
- No [specific creature/spell/weapon] and could be biased towards cards like Miracle or Novas or something
- Use at least X of [card] where the card is not a pillar/tower/pendulum and X is less than or equal to 6
- As you suggested, the number of cards in your deck should fall in a certain range
- Use a particular mark
And say the bonus was, for all 3 conditions met, your Arena Deck would get +10 HP, as though you spent a couple of skill points towards Hit Points. Realistically, the bonus wouldn't be an extra skill point since point spending is done before you build your deck; from a programming perspective, the addition of these conditions is difficult enough without complicating the deck-building phases.
This would be a good balance since it doesn't affect the Arena opponents too much, and it achieves the goal of encouraging creative deck-building, albeit with a reward. Though in all honesty, the Arena just seems like a way to make money. Creating incentive for the submission of original decks seems a challenge in itself.