Recoup

Most flashback gives a single card a second cast; this hands the keyword to whatever sorcery is already in your graveyard, and lets the flashback price be that sorcery's own mana cost rather than a fixed surcharge. That distinction is the entire engine. Recoup is a buyback-from-the-yard for spells you have already paid for once, so the dream targets are the expensive end of your sorcery sheet: a Mind's Desire, a Living Death, a Wildfire, anything whose effect dwarfs the two mana it costs to re-enable it. Cast the sorcery, let it hit the bin, then spend Recoup to re-arm it at face price plus the price of Recoup itself. The card also has its own flashback, which closes a small loop: it can be cast a second time from the graveyard to grant flashback again before it exiles itself. The tension Wizards built in is that you are always paying twice, the original cost and the rate to unlock it, so Recoup only earns its slot when the spell it revives is doing something the second copy could not do cheaper. It is less a card than a verb applied to your best sorcery, and the upside scales with exactly how degenerate that sorcery is.