Leyline of Anticipation
Granting your whole deck flash sounds like a build-around and plays like a license to break the rules of sequencing. Every creature becomes an ambush blocker, every planeswalker arrives at the end of an opponent's turn, every sorcery-speed enabler waits until it can be deployed with information the caster didn't have when the turn began. The pivot is the timing axis: it does not change what your spells do, only when you are allowed to do it, and that conversion of static cards into reactive ones is worth far more than the four mana implies. The Leyline shell is the real trick. Free deployment off the opening hand sidesteps the only genuine weakness a four-mana enchantment with no immediate impact would have, namely that it does nothing the turn you spend tapping out for it. Drawn off the top it is a fine if unexciting fourth-turn play; sitting on the battlefield before the first untap step, it warps the entire game from the opening exchange. That conditional zero-cost start is the lever the rest of the Leyline cycle shares, and it is what separates this from a card that simply reads "your spells have flash." The ceiling is an opponent forced to play every turn as though you are holding open mana, because you genuinely could be casting anything at any time.
