Being able to play whatever you want without worrying about the cost, or plumbing all that mana into an obscenely massive X-cost spell, can devastate the board
which means we,the players that don't blow their money on "random chances", will also have to buy checklist
Like Return of the Wildspeaker, you can pay five mana (if you foretell it) to draw a ton of cards if you have a big enough creature, and like Inscription of Abundance you can gain lots of life while you're at it
While Undermine and Absorb are roughly equivalent in how powerful they are, Absorb edges its way forward due to the life gain aspect
Personally, I think it might be the absolute best
What abilities would it have