Some people are calling it "the end of the game as we know it." Indeed, it bears a striking resemblance to something I "predicted" in World War M, the fictional article I wrote in 2012 about what it would take to kill Magic
Offering some cross-tribal benefits for assassins, mercenaries, and rogues, Mari seems like an excellent support card for Etrata, the Silencer due to her ability to exile enemy creatures with hit counters on them when they die
It also grants you protection from everything until your next turn when it enters the battlefield, which makes it a good target for blink effects that can trigger that multiple times
In a 60 card deck, wed have three shocklands of each color and a basic, so we have sixteen sources of each color, which is to saw that there are sixteen different cards that could be in our opening hand that would allow us to have green mana, for example