My all-time favorite had to do with the card Yawgmoth's Agenda from Invasion
To clarify that, there were turns where I could play something to win the game (or to not lose the game), or I could play Consecrated Sphinx because I wanted to draw some cards
Vault progression seems fairly slow process with common providing only 1 point and uncommon 3 points (ref)
Minamo Scrollkeeper, Trusted Advisor, Doctor Octopus, Master Planner, Cecily, Haunted Mage/Eleven, the Mage, and Twenty-Toed Toad each increase your maximum hand size to a specific amount and might fit your deck's plan
Wizards previously teased the inclusion of iconic weapons such as the Vorpal Sword, along with dragon villain Tiamat - whose card features all five of MTG's mana colours, relating to each of her heads
Part of the original 151 Pokmon from the Kanto region, Gloom evolves from Oddish at level 21 and can transition into either Vileplume with a Leaf Stone or Bellossom with a Sun Stone