Planechase Plane cards are oversized land cards, measuring about 3.5" x 5" (9 x 12.5 cm)
The main thing is, if a cube is too large (500+ cards), the discrepancy between individual card power levels is going to be very high
Dont fold to one card: If a single hate piece turns off your entire deck, you are not focused. You are brittle
Better access to black mana just lets us protect Niv-Mizzet with better cards like Thought Erasure, and Im sure Crackling Drake doesnt mind a partial cantrip interactive spell
It would have ended up just being a big pile of vampires without much synergy and who wants that
Throw in Witch's Clinic, Hedgewitch's Mask, Witch's Vengeance, and Witch's Cottage for some variety in your spell and card types and youre set