Historical Headcanons? Any characters really, whomever jumps in your mind. Any era too although personal I adore the 16th-early 18th century and WW1 and Weimar Germany.
Holy Rome during 1600s especially thirty years war with all that infighting n stuff and high death toll and "scorched earth" policy?
Oh boy I have a ton of historical headcanons, so I'll just do a few shorter ones based around the characters I have drawn on my blog so far, HRE and Prussia, and the eras you mentioned. These aren't particularly deep but I hope you still like it!
HRE is a young adult (17-18) in my headcanon around the late 1500s onward, but throughout a majority of his adult life, his health was constantly hanging in the balance. HRE as an actual country was always being pulled in different directions; as far as I know, it was hardly a functional Empire. So that definitely reflected onto HRE.
He always knew he was going to die slowly, and did his best to ensure no one knew he was in pain/coughing up blood. His health took a definitive nosedive in the 1700s, though he was a stubborn mule and refused to remain bedbound despite Prussia, Veneziano, Hungary and Austria trying to get him to stop. It became clear he was going to die when most of them (who HAD been younger than him physically) surpassed his age.
They didn't succeed and he died on the battlefield, despite the wound not being particularly deep. Prussia was present.
He never really had too many portraits done of himself, despite his wealthy estate, so Germany doesn't really realise how similar they look.
HRE raised Prussia! He was raised until he had to join the Teutonic Knights, which HRE totally forgot to mention he represented. Oops. Over time, he became Prussia, and tried to be HRE's right-hand-man. He was very close to HRE as a result.
Prussia went a little crazy after HRE died. Both just going off the wall for many years, but also desperately wanting to bring his brother back. Then Germany was... born! Probably not related at all.
The Weimar era was super interesting but one main thing that always stuck with me about it was that it was pretty gay. Lots of gay bars/clubs/etc in cities, underground for sure, but an open secret. It was definitely an era of exploration before politics took a sharp turn. I would consider this era to be Germany's (as in, Ludwig's) gay awakening.