HMMM love this discussion.

Clearly there must be more Primals than elements as there already are explicitly with the implication of Bahamut.

So What I'm getting so far is:

the possibility of Repeat elemental Primals, Polarity Primers (dark/light)
non-elemental or multi-elemental Primals.

To add my own thoughts, I would say that it was never implied that a Primal absolutely 'has' to have some elemental alignment. Though I think an alignment of some kind is implied simply by their nature as deities representing aspects of the world.

I think someone like Bahamut is not an elemental Primal but a Dragon Primal (this is an alignment with a kind of beast not an element). At the same time I also feel Bahamut is Non-elemental.

Odin is tricky to pin down, But I would lean towards Polarity (dark primal) Who knows maybe even a human-kind Primal (elezen, lala, roeg, hyur, miq