[Update]: MTG Arena's Dominaria United set has now arrived alongside the annual Standard rotation.
Once it arrives this September 1st, Dominaria United will bring with it a whole bunch of new cards to mess around with, as well as the annual rotation that will see Zendikar Rising, Kaldheim, Strixhaven and Adventures in the Forgotten Realms sets leave Standard legality. So if you ever wanted to give MTG Arena a try, Dominaria United's launch would be the perfect time to do so as you'll only have a couple of sets to worry about.
While the full trailer has not been released just yet, you can get a pretty good idea of the sort of story beats and atmosphere Dominaria United is going for through the brief teaser below. Have a gander:
As for the cards themselves, there will be three new and two returning mechanics for you to build decks around: Enlist, Read Ahead and Stun Counters, as well as Domain and Kicker. Here's a brief rundown of what each of them does:
Enlist allows an attacking creature to tap down a friendly creature without summoning sickness in order to gain +X/+0 until the end of turn, where X is the power of the enlisted creature. Since some creatures in the set actively want you to tap them, Enlist should work out pretty well in limited formats.
Read Ahead gives sagas the ability to enter the battlefield on whatever chapter you want. Doing so will make you lose out on some value as you will skip the previous chapters entirely, but having the option is certainly welcome.
Stun Counters keep permanents (mostly creatures) from untapping. Basically, if a tapped permanent with a Stun Counter on it would become untapped, instead it stays tapped and you remove a stun counter from it. This seems like a new twist on the fairly old tap-down mechanic, so I expect Stun Counters to play a big role in limited formats.
Kicker is a returning ability that allows you to pay a bit of extra mana while casting a spell in order to get additional benefits. The only slight difference this time around is that most of the cards have off-color kicker costs, thus encouraging splashing and multi-color decks.
Domain is another returning mechanic that gives you benefits based on the amount of different basic land types you control (Plains, Island, Swamp, Mountain, Forest). The new tri-colored lands from Streets of New Capenna count as three basic land types all on their own, so I expect Domain decks to at least make a bit of a splash in Standard.
If you're curious how all of these concepts translate into actual cards, as well as what sort of theme each color pair is going for, you'll be happy to hear that the full set has now finally been revealed! As such, you can check out the entirety of Dominaria United over at the official card gallery.
Once Dominaria United goes live I'll make sure to let you know. Until then, you can learn a little bit more about the set, as well as read some of the short stories, over at the MTG website. Enjoy!