Different possible tunings

Hi all, I’ve experimented with some different modes (is that right? I’m meaning DAd, DAa, DAc) as well as those same relationship of notes in CGc etc.

i was wondering if there was anything other than “A” for the middle string to be? Or “D” for the bass string for that matter?

This mode stuff is completely new to me and I’ve been playing guitar for 20+ years!