Who do you think the original 13 Divisions were? The only members we know from Canon are Yamamoto and Unohana.

I think that Ichibē and Ōetsu were two of the other members. Ōetsu would have to be, right? Since he created Zanpakutō. And Ichibē created names/named everything in SS, so it makes sense for him to have been one too.

Another possibility is Sajin's great grandfather.

I also think it's possible for 5 of them to have been from the main Noble Houses, possibly the first heads of those houses. Shiba, Kuchiki, Shihōin, and the two we don't know the names of. After all those nobles do refer to themselves as "The True First".

That would leave 3 of them I don't have ideas for.