The only buff Kayle really needs

Just like, restore 50% of her mana used on her E if you target and kill a minion with it. (I say target and kill because it is AoE post-6, so no double-dipping by attacking champs with it). Her atrocious early game would be fine if she could at least do what you implied for her to do in her release video: Use her abilities to farm minions safely from range. As it stands, she'll OOM in a wave or two and be a sitting duck in lane praying her opponent is too stupid to act on it.

Annie and GP both got this out of necessity, I don't think it'd be too much on Kayle either, given Annie can 100-0 you and GP is a viable fighter at all stages of the game.