Ah ... Zephyr!!! I'm so dumb, sorry :(
The big thing about Janna is that she is inspired from Storm (one of the X-Men), a person who manipulates the weather (in case of Janna, the wind) to do whatever she wants. If there would be a Wind Elemental in this game, it should have different techniques to control the wind, or it will be compared with Janna constantly in terms of power.
My second thought would be that it's lore would be difficult to be created (I mean, he shouldn't be another mystery of LoL which appeared out of nowhere with no real reason). Also, I think there are too many users of the wind (Riven, Yasuo and Janna). The two sword users don't really use it as their main weapon, but they enchant their attacks with it, and they even use it to defend themselves.
About the Water Elemental, I don't really know what to say. In terms of Lore, it can easly be considered a monster of the deeps who wants to make the whole world an ocean (generic, but it works xD). I would be interested to see how its kit should be though. That is the only big problem I think.