Could it be possible that Tobirama is at least part Uzumaki?
I mean, from everything we've seen, He fits Uzumaki more than Senju
White hair, like the former clan head
red eyes, like Karin
possibly good with fuinjutsu? (Edo tensei, whatever his S/T tech was, making Raijin "Thunder sword"?)
just a random thought i had, feel free to bash.