[Update #2]: The Lost Caverns of Ixalan has now arrived with dinosaurs, vampires, bonus Commander cards and a surprisingly aggressive limited format.
[Update]: All of The Lost Caverns of Ixalan cards have now been revealed ahead of the November 14th launch. And, Brawl players are getting a little bonus surprise!
If you're a fan of the Ixalan plane and its weird mixtures of dinosaurs and vampires, among other things, you'll be happy to hear that MTG Arena will soon be making a return! Once it arrives this November 14th, The Lost Caverns of Ixalan will bring with it around 300 new cards to mess around with, most of which are focused around the concept of exploring a long-forgotten underworld.
In terms of new mechanics, there are three you should keep an eye out for: Craft, Descend and Discover. First and foremost, Craft is a new way of transforming artifacts. By paying the craft cost and exiling a specific number and type of cards from the battlefield or graveyard, you can transform your artifact and get a powerful effect.
Descend is a mechanic that cares about permanent cards in the graveyard. Depending on the card this means you'll be getting some sort of effect when permanents enter the graveyard or purely based on the number of permanent cards in your graveyard. The most important thing to note here is that Descent does not care about tokens hitting the graveyard as it has to be a permanent card!
Discover is essentially a new version of cascade. When Discovering X, you exile cards from the top of your library until you exile a card with mana value X or less, and then you either cast it or you simply put it into your hand. Afterwards you put the other exiled cards on the bottom of your library.
What all of these mechanics look like on actual cards, that you can check out through some of the spoiled cards over at Scryfall since the official MTG website is unfortunately quite messy. As for the foreign cards, simply click on them to get the unofficial translation. Or, if you don't feel like going through the hassle, wait until November 3rd as that's when Wizards of the Coast will be releasing the full card gallery.
Once that happens, and when The Lost Caverns of Ixalan set goes live, I'll make sure to let you know. Until then, give the various short stories a look if you're curious about what's even happening with The Lost Caverns of Ixalan. Enjoy!