Hearthstone's next expansion, Voyage to the Sunken City, launches on April 12. The new set will introduce an entirely new class, Naga, the undersea race of magical beings, which reward using spells while they sit in your hand. When you get a class-based legendary Naga, of course the spell-based synergy is huge.
Enter Hedra the Heretic, a Druid legendary minion and part of the new Naga class. The 7-Mana minion with 4/5 stats has a relatively low stat-line, but its Battlecry is huge. It summons a minion equal to the spell cost of each spell you've cast while Hedra was in your hand. Yikes.
That ability makes Hedra a good late-game finisher for Druid decks that favor big spells. Some of those will be cycling out of Standard rotation this year, but last year Blizzard introduced the Core Set, which allows it to cycle in cards from any past expansion to lay the groundwork from upcoming expansions. It's safe to say that we'll either see some high-cost spells returning, or new ones added to the pool in the Sunken City expansion.