I'll rate from best to worst out of stuff I feel could have potential but Riot will never let happen:

Taliyah is fully capable as a jungler. Blue reliant but it's manageable. She's just way better in a lane.

Basically another Diana and we see how popular she is. Also performs much better in lane.

Leona is a jungler that's basically already usable but is too unreliable at the moment due to the fact that when she falls behind she falls behind hard. Her farming isn't bad at all with right runes/masteries and is a ganking machine. She'd just need 1 or 2 tweaks to be insane.

Good ganker, bad farmer. Even with offensive runes/masteries your early clears are pretty bad and abusable. Still very fun, and clearing becomes easy once you get Bami's.

Soraka would just need damage buffs in the jungle. Her clearing is safe and healthy, it's just god awful slow past a certain point since her damage doesn't really scale up with anything. Workable as a pick in norms but too abusable to be a ranked pick.

Sona has the ganks, but her insane blue reliance that isn't offset by good clearing, in fact her clearing is awful until runic and that's not gonna happen quick enough.

Basically troll but still funny when it works. Pre-6 is awful but post-6 she's the fastest jungler in the game.