In my experience Keplin is easily the weakest of the champions. The main reason in my mind is his channeling tree. I almost never find myself using more than 2 or 3 casts in a fight, and early game I usually don't have the mana even if I wanted to. I really like him thematically as the champ that aids sovereign casting, but I think there are more effective ways to do it than simply adding sovereign casts. Here are some ideas for abilities that could replace some of his channeling line that would make him more fun and useful:

1) On Keplin's turn the sovereign can cast twice

2) An active ability that grants all allies 30% spell mastery

3) -50% mana costs

4) +50% damage from damaging spells

5) An active ability that brings all sovereign spells off cooldown

6) An active ability that lets him take another action and makes the mana cost of the next spell cast 0