Hecarim needs to be buffed IN A DIFFERENT DIRECTION

You can't buff in general, that won't work. At this rate to make him decent for jungle you're going to end up making Maokai 2.0 where you try to make him a viable jungler but just end up making him a god top lane. Don't mess with the mana costs, attack the problem at its root; just make his Q do normal damage to minions and monsters. It won't hurt anything. He'll still have average clear time and average ganks.

As for Skarner, Elise and Evelynn, they're another story. They might need more to be good in the new jungle (and right now, a big part of them isn't themselves, its the BS mobility meta you've made). But Hecarim, he just needs normal Q damage on minions and monsters. This isn't like previous season where monsters are little inconveniences that give gold and XP, right now to clear the whole jungle Hecarim needs a lot more mana than he has.