Increase rewards for limited as a whole.
Improve the rewards from the Cube draft. Arena is already enough of a cash grab. As it is now, you don't even pay out the -already paltry- rewards it's supposed to give. There's literally no reason to pay ANY gems, nor 4k gold to NOT get to keep the cards, and get next to nothing for participating, even if you win out. You get more from just buying 4 packs with your gold.
Heck, for that matter, improve the rewards from ALL limited play modes. Right now, it's more beneficial to NOT play the game, and instead just buy packs with your gold from dailies, than it is to even try limited - especially considering you're going to draw 9000 lands every game.
This is an incredibly disappointing realization, as the only reason I came back to Arena was for Limited, and even THAT is an unbalanced cash grab.