Scenario Simulator
As a long-time absent player returning to the fold and trying to teach a friend how to play, it has been a bit overwhelming trying to not only catch up with all of the new rules but to decipher exactly how specific card interactions would work. While trying to do this, there have been multiple times where the interaction of cards isn't very clear, and the end result is left up to a fair amount of speculation. We then turn online, hoping for specific rulings on the cards in general, but if one doesn't exist, "some guy on reddit" might be the only available resource, and if the outcome is especially painful for one party, leaves us not wanting to put full faith in that answer.
So I think a interesting and super helpful new feature, would be a one that allows a user to build a specific battleground state, dictating all of the cards in play, and in other locations (library, graveyard, exile) with as little or as much detail as desired, and run through card interactions with the outcomes shown as they are while playing the online game. This could act as a guide for people wanting to play in real life, and would allow for players to have the simulation help players determine the end state of combinations of cards acting off of one another. Even if its not considered an end all be all judgement for it, it would still act as an impartial arbitrator to allow someone to determine a ruling, with no implicit bias.
This can also be useful resource for those wanting to deck build in the virtual game to test specific interactions to ensure they play out the way they think they would.