Right now Magic: The Gathering Arena is in its beta phase for an "undetermined amount of time" before it's actually unleashed upon the world.
As such it's currently running on cards that are cycling in and out of standard play in the real world version of the game (with the exception of Kaladesh and Aether Revolt) -- and that means new expansions are set to arrive at the same time as their paper counterparts.
Dominaria, the next set of cards, just launched yesterday in Arena. You can now buy Dominaria cards for 1000 coins a pack, or win them as part of the new set of weekly rewards (Rivals of Ixalan has cycled out of the reward list). Right now tech for the new expansion is very early, but I've seen some really mean decks involving Teferi, Hero of Dominaria and other Planeswalker cards. There's also a few smaller sleeper cards in the set I'm looking at trying out in the paper version and Arena.
I can dig this single release cycle!