Late to the party, but I like your take. I'd been thinking about the Four Founders for a while... and then J.K. went off the rails and I felt uncomfortable actively doing anything Potterverse-related. I've been gradually getting back to it, though.
One of the paths my thinking led down was that Hogwarts was
founded in the late 800s/early 900s, but we don't really know how old the Founders were at the time. The Anglo-Saxon encroachment into Britannia had been going on for several centuries, but the language was really only solidly established by about the late 700s. Godric, Helga, Ravenclaw, Hufflepuff, and [Slitheren] are all Old English/Anglo-Saxon/Germanic. Rowan is Gælic. Gryffindor is, well, messy. Best etymology I can run on it makes it likely Greek. And Salazar is Basque, so that's a thing...
Anyway. We know the school is in Moray, in the Scottish Highlands, near where the town of Dufftown was established in the 19th century. That region was unified by the House of Alpin into the first unified Kingdom of Scotland in the 9th century. Right around or a bit before when the school was founded. Rowling's lore says that the founders picked the spot they did to place the school far from Muggles, because of their fear and persecution of witches. Welllllll... There's been activity in the region since at least the 6th century, and most people of the era in Britannia were pagan and valued their witches and wizards. So there's
that.
So all of that led to me pondering where the founders came from and why. I can see Helga being descended from Saxons who came over over the course of several centuries. We know Godric and Salazar were friends prior, despite their differing views on Muggles. I can see both of them being around in southern Anglè-Land, and encountering Helga there. Rowena seems more likely to be from the North, possibly of mixed Gælic and Anglo-Saxon descent.
So I feel like, while Helga and Rowena likely originated on and stayed on or close to the island of Great Britain, Salazar and Godric may have spent more time on the continent. Not necessarily
from Europe proper, but with more connections there. So Godric's sword might have been just about anything decent or better-than-decent of the period from the 7th to 11th centuries (I give the Wizarding World credit for being a little ahead of the Muggles in certain artisanal respects during this period). I like the sword you used for inspiration. There were two I had narrowed my thinking down to...
If we were to want to create something evocative of the sword from the films, but be a bit more historically accurate, we could use this one as a starting point. There are numerous contemporary replicas of varying levels of quality. The one pictured is one of the best out there. The original, from what I've been able to dig up,
seems to date to the 10th century, but I can't find verification of that. It's a sword to honor King Solomon, so it might actually date a couple centuries later, to the First Crusade and the Crusader Kingdom(s) in the Middle East...
If we wanted to stick with a more Northern-European origin, the best one I've found is a replica Viking sword of a style in use from the 7th to 11th centuries...
Either of these, I feel -- with nicer, "goblin-wrought" furniture and the rubies and all that -- would be good candidates for what Godric would have carried... but nor for duelling, as J.K. says. Duelling with swords wasn't really a thing in Europe until the Renaissance. And, from what the books (and, later, films) establish, wizards duel with magic, not swords. So I would not expect him to have a duelling sword.
The one you're making is gorgeous and appropriate also. I appreciate the thought you've also put into it and wanted to share my ruminations.