I think there need to be a couple banned cards in arena, namely Supernova and Quantum Towers. The problem with them is that people have decided that they'll just toss whatever the oracle card is into the same old rainbow deck because it can handle some dead cards. The result is that at least half the decks you play against are basically the same thing.
If Supernova and Quantum Towers were banned, it would force people to try and use their cards, or at least give a bit more variety.
Different solution proposed by majofa.
Just a thought. When the Oracle spins it lights up X amount of elements. You can only use cards of those elements in your deck. (Any MARK) ... X could be anywhere from 2-12. The element of the card you get is always lit up.
(forgive my poor editing skills)
Thoughts on this?
Picture of how it might look by Xenocidius: