Wow. Very interesting mechanic in black. It'd more normally be seen in green or white, but if this set has Curses or similar then it's somewhat plausible in black. I'd expect it to be uncommon rather than common though (especially as it's bypassing mana cost: turn 1 cast whatever the set's 1-mana discard spell is to discard Omniscience, turn 2 Bloodthrone Vampire, turn 3 this and win?)

Ah, sensible tweak. Auras with terrifying abilities and prohibitive mana costs are much less frequent than global enchantments like that. Now it's only really Eldrazi Conscription you have to worry about.