Doomfall is an extremely flexible, potentially main-deck, card.
Meet the Cruel Edict
Doomfall costs one more mana than Cruel Edict, but even beyond its ability to act as a discard spell, there may be good reason to pay three instead of two.
Rather than merely destroying a creature (as Cruel Edict did), Doomfall exiles it. That is bad news for Scrapheap Scrounger decks! Any creatures that rely on durability or coming back from the dead might have problems with it (e.g. The Scorpion God)
Playing as Transgress the Mind
Again, we see a card that, at 2b, costs a little more than Transgress the Mind. Part of that is just a tax levied against the creature removal side of the spell (you can’t very well have a functional reprint of Staple Transgress the Mind that also kills creatures).
But the reality is, Doomfall is not “just” a reprint of Transgress the Mind. It costs one additional mana, but on the other hand you can take anything, rather than only cards that conform to a particular mana profile. That might not feel like it’s worth three mana in isolation… But you’ve always got the Edict opion!
Ultimately, it Can Get Everybody
This card lets you beat a combo deck with creature removal, or kill an attacker with your extra Duress.