I think I know what it is...
While EB is hardcore about historical accuracy, it is not impossible for them to have a little fun.
All the possible historical factions have been shown, I do not think that it can be the Indians because EB map simply does not cover all of India.
If it is just another obscure historical faction, EB would not keep it a secret like this.
My guess... the Amazons or some odd rebel faction.
Remember Themyskra (spelling)? If CA had the liscense to have its fun there, why not the EB team?
Look carefully at Khelvan's signature, the part that is uncovered near the "C" shows tips of green lettering. I would pull it into photoshop to examine it more closely, but I am fairly certain that it is a letter without any loops...