There are a few ways we can look at this:
The first possibility is ofcourse the element he learned first, which would be Fire;
The second possibility would be what @Voldemort suggested: the last element he learned, which would be earth.
The third possibility is something different altogether: it would be the last element he used before he took Ravah inside him (note: just inside hime, the permanent fusion). Following Voldemorts theory, it would be the element W, which he was holding inside him, while Ravah then held elements X, Y & Z.
The last possibility is closely related to the third one: here however, it would be the element he used last just before the permanent fusion. In the previous theory, each time he switched while Ravah was inside him (but not permanently bonded), you could argue that Ravah simply switched Wan's element W with her X/Y/Z element. So the last element Wan held (his W element), would become the first element of the cycle.
The easiest way to determine this would be to find the second avatar and get their element, because it's mostly guesswork at this point.
Personally, I find either #1 or #4 the likeliest.
EDIT: Thinking about it, the logic in the OP could be correct too.
There is always a cycle that has to be followed:
The avatar has to learn the elements in order of that cycle, starting with his own (nation's) element.
And Wan first learned fire, so you could reason that makes him a firebender.