AP melee hydra.

Gragas is already all AOE in his damaging abilities, plus he has sustain to allow multiple uses of them, and damage resistance to also help in his clear. So it would not make a whole lot of difference on him and might also become a bit more OP on him. But just imagine champions like with an item like that. They would become super op too.

Hydra items usually are meant to help those more single target AD melee champions wave clear and deal some damage in fights to multiple targets, especially if they are meant to be limited in range/mobility and will find themselves in the center of fights having to fully commit. Most AP mage champions already have great AOE damage and a good amount of wave clear. They also tend to have at least 1 long ranged ability or some good mobility (sometimes both). So an AP variant of the hydra items is not necessary.

Already tried an AoE Lich Bane-type item for Junglers. It didn't work out.

I tried the lifesteal one on the old akali once, it was meh, but I'm not a great akali player. Jungle Eve might benefit, but I don't see it built on her. Her Q gives her enough AoE damage that you'd consider the items more for their stats than anything. Leona can make use of Titanic Hydra as a damage option if going jungle or top... in fact I think she needs at least one damage item if going into a solo role.

Hydra doesn't work with double hits, so shyvana's Q or Eve's E don't proc it twice. Just and FYI.