[Update]: The Shadows Over Innistrad Remastered expansion has arrived with hundreds of classic cards and a really fun draft environment!
In order to really spice up both the Explorer and Historic formats, MTG Arena will be launching Shadow Over Innistrad Remastered on March 21st. The remastered set will bring with it 302 classic cards to mess around with, as well as an additional 81 cards from the original Innistrad set that will be exclusively available in Historic.
I'd say the definite highlight of the new set is Emrakul, the Promised End (pictured below). It might not be the most powerful card around these days, but it's easily one of the most unique cards on Arena, and one that's so complicated to actually implement that many feared it would be skipped over entirely. I can only hope it doesn't end up being too buggy because there is an insane amount of stuff you can do when you have full control of an opponent's actions, especially with all of the new digital cards.
Emrakul sure is a doozy of a card!
As for the bonus 81 cards, these can be found as extra little treats in booster packs, as well as a 'Mystical Archives' style addition in limited formats. However, these new Shadows of the Past cards will follow a bit of a different formula than their predecessors.
Instead of being available all at once, each week will bring with it a new batch of extra cards that will appear in limited formats. Just to further clarify - limited formats will be getting a different batch of cards to mess around with each week, while packs will contain all 81 Shadows of the Past cards from the get-go.
You can check out the full list of these cards, as well as which order they will appear in, over at the Shadows of the Past card gallery. As for the main Shadows Over Innistrad Remastered set, you can find all of its included cards by hopping over to its respective card gallery.
Once the Shadows Over Innistrad Remastered update goes live on March 21st I'll make sure to let you know. Until then, have fun brewing up new deck ideas as there's a lot of wacky cards on the way!