Thunder: I would choose Xerath. Kennen would be my runner-up choice, but ultimately I think Xerath's presence and mastery of the element are more commanding.
Fire: I would choose Brand. Brand's lore establishes him as a sort of fire elemental, whereas I think Annie's use of fire is possibly just a chosen affinity; Annie isn't a fire creature, per se, and making a familiar of Tibbers indicates the breadth of her abilities. Brand is just all fire, all the time.
Earth: I agree with your Malphite choice, though I would make an argument for Galio (he's a gargoyle who makes stone jokes!). Disclaimer: Galio is my absolute favorite champion, and I wish like hell he got more love. So Malph might actually be a better fit, but truthfully? If you picked Galio, you'd be the coolest person ever. No joke.
Ice: This is a tough one....so many to choose from. I would say Lissandra with Anivia as a very close runner up. Lissandra has a lot of potential in fan art because let's be honest, she's super creepy. (She seriously does look like something H.R. Giger created.) I can't offer much argument over which of these two is a better representative of the element itself because I think they're both good; so you should decide this one based on which one offers you the aesthetic you're going for, imo.
Wind: My pick would be Janna. I prefer Yasuo as a character, but I think Janna is more defined by her affiliation with the air element, whereas it's more like a plot point for Yasuo.
Darkness: More excellent choices here, but I'd pick Nocturne. His whole characterization is based on actual darkness (ie, the absence of light) and not just darkness as a concept. His body is darkness personified, his ult darkens the map, his taunts and emotes are based on darkness and nightmares (which, as we all know, are associated with sleeping and nighttime and....darkness!). I think of the others as having darkness as part of their overall aesthetic, but not necessarily being BASED on it. Nocturne, I think, is really BASED on it.
Light: It's a toss up between Leona and Lux, but I would ultimately choose Leona. I think both champions get a lot of attention in the fan communities, so either one would be a good choice in that regard. Leona is part of a tribe that worships the sun; Lux, while also having command over the powers of light, is more of an anomaly; she just kinda happens to have this power, whereas Leona belongs to a clan where light-reverence is a way of life.
Those are my picks! Feel free to disagree and go with other champs, but I hope this input helps you decide. =]
