This is just a guess.....
But it does have some basis in fact.
Tobirama to Hiruzen
Hiruzen to Jiraiya(originally, but he refused) so it went...
Hiruzen to Minato
Minato died, Hiruzen had no choice but to reclaim leadership.
Hiruzen to Tsunade
Tsunade to Kakashi(Stand in Hokage. Almost officially became Hokage, but Tsunade recovered.)
Tsunade to Naruto(It's obvious)
The Hokage-ship of Konoha has always stayed within the Hokage's students and teammates.
So I believe that Hashirama's teacher and possibly the previous leader of all the clans before Konoha was created and Hashirama came into power was none other than...
But it does have some basis in fact.
Tobirama to Hiruzen
Hiruzen to Jiraiya(originally, but he refused) so it went...
Hiruzen to Minato
Minato died, Hiruzen had no choice but to reclaim leadership.
Hiruzen to Tsunade
Tsunade to Kakashi(Stand in Hokage. Almost officially became Hokage, but Tsunade recovered.)
Tsunade to Naruto(It's obvious)
The Hokage-ship of Konoha has always stayed within the Hokage's students and teammates.
So I believe that Hashirama's teacher and possibly the previous leader of all the clans before Konoha was created and Hashirama came into power was none other than...
The Sandaime Hokage's Father, Sarutobi Sasuke!
The man Uchiha Sasuke is named after.
The man Uchiha Sasuke is named after.