Electric is my first choice. As mentioned, Ground weakness can be remedied by non-Levitate ways (Such as Magnet Rise. This is an utility Pokemon after all. ) Electric/Poison designs are also very promising
Dragon is my second call. Putting aside its ridiculous amount of resistances (And why doesn't anyone advocating a Levitating Poison/Electric mention Thick Fat on this guy?) design ideas are kind of limited, kind of not. While it will be a poisonous dragon/serpent, we have a lot of those, like basilisk, Vasuki, Jormundgard, Nidhogg, Typhoon, Yamata no Orochi (A drunk Poison/Dragon!), Hydra...
Note, those favoring Steel/Poison, by the way, an alchemical legendary trio was one of my ancient ideas - mercury, salt and sulphur Pokemon. In alchemy, mercury symbolizes the spirit as opposed to salt's body and sulphur's soul, and is one of the major components making up the human body. We can use this to give our Poison/Steel a mystic aspect and allow it utility Psychic moves like Gravity, Trick Room and whatnot, and perhaps it can flavor the Pokedex entries.