Okay, I find one of the problems with creating an Arena deck is that the AI is very, very stupid and so there's decks that you just can't reasonably put up.
I've just won a
Nymph so I thought I'd go for a trio - Nymphs, Antimatter and Adrenaline. The strategy, of course, is to Antimatter, Liquid Shadow and Adrenalise your opponent's creatures so that they heal you, hurt them, and die. The AI, of course, only uses the Antimatter on its opponent's creatures and instead uses Liquid Shadow and Adrenaline on itself. I've left the deck up because I think it's a nice idea, but it's not going to win, and the main reason it's not going to win is because the AI is stupid.
So my suggestion is to have the ability to link cards with other cards in your deck, or de-link them. So, for example, with the deck above, I'd be able to de-link Liquid Shadow and Adrenaline with my Nymphs, and that would prevent the AI from playing either of those cards on my Nymphs. Or, say, if you had a deck which relied on having Lobotomiser and you also had several Electrum Hourglasses which weren't as important, you could link Protect Artifact to Lobotomiser, meaning that the AI wouldn't play it on anything other than Lobotomiser, rather than Protecting your Hourglasses, as it normally would.
I'm not sure how easy this would be to implement, but it'd make the game much better.