All champions should use mana to make the game more balanced.
Energy should not be in this game. All champions should use mana to create more balance. It's a very simple idea.
As a mage I have to often back just to get more mana and early mana generating items. I used to buy a fiendish codex first but now i have to buy a mana book early. Or a second Doran's ring to supply me with the mana I need to lane and harass effectively.
Zed, Yasuo, Riven, and many others don't have to face this issue. Take away this idea of energy and rebalance your entire game as all champions using abilities must use mana to operate them.
This will change the game a lot very quickly and the game will be much better off for it.
If Sivir and Varus and other ADC's are gated by mana then so should the fighters and bruisers. Everyone in the game should be gated by Mana.