It's alright, we feel your pain. Noone wants to admit being gar for Berserker.
I have always perceived her element as fire-the whole'descended from dragons/has dragon blood' which i thinkn is supreme awesomeness(idk if thats a word or not). I could imagine her being void, after 'anti-(insert what is being anti-ed here)' is her thing. Also having the same element is shirou would make sense... This isn't mentioned rly, as she is a magic user(she uses reinforce and pranaburst) but not a magus, so i'd like to know what element you think she'd be.
It doesn't really matter for people who don't use magecraft. In the case of Arturia, I would say that Avalon, and probably also her swords, are what bestowed her with magical powers. Being fused with Avalon is also what allows Shirou to develop his powers, and is the origin of his 'sword' affinity. Arturia originally was just a powerless girl who wanted to help her country. After picking up the swords and Avalon, she became their vessel. If I had to guess she has the 'sword' affinity like Shirou.