Like Seriously, Gate Problem Mages' Mana Already
You want a mage that can handle top lane? Cool! Gate his mana. You want a mage with extreme burst, range and mobility? Awesome! Gate her mana. You want a mage that has the longest and longest ranged AoE hard CC in the game that is also a nuke? Don't! But if you do, gate her mana too.
Breaking the rules should cost something, eh? Wouldn't it be fitting if it were a...mana cost?