Bard Bard is about as good as it gets. :P I'm sure they're having a hard time following that one up.
Sion actually has a decent amount of skins, and I'd be more interested in them reworking a couple of them (specifically Hextech Sion, whose art and model look almost similar to his normal skin other than using a Maul instead of an axe in game)
Skarner and Rek'Sai are both pretty difficult to make skins for, though personally, I think Skarner needs an iSkin. Similar to iBlitzcrank - make him really sleek, and give his ult an almost UFO sort of flair by making it look kind of like a tractor beam or something.
Someone already mentioned this with Morde, but now that he's getting his rework Sparing You, I'm down with seeing him get a PROJECT skin. I think it'd fit his kit and abilities really well, and they can give everything he does an eerie glow.
Also on the boards somewhere is Atlantean (or Deep Sea) Zyra that summons aquatic plant life like seaweed for her snare and ground FX, anemones (yes, I had to use spellcheck to spell that right) for her E-plants, and spike-shooting urchins for her Q-plants (cheating a bit because urchins aren't actually plants...but hey, it was a great freakin concept.)
The most recent one I was moved by was Zac & Cheese (going along with the Baker Pantheon, Sashimi Akali, etc...) because that sounds royally amusing to go picking up little macaronis every time you use an ability.