Assassin's Creed Mirage largely succeeded at being a return to the franchise's roots. Combining some of the newer elements with more stealth-based gameplay and quicker action managed to impress us and other fans when it launched in October last year.
In a recent Reddit AMA, it seems that even if Mirage is a complete experience, it might not be the end of Basim's story. Stéphane Boudon said the following in regard to pursuing Basim's story further.
"Mirage [h]as been design[ed] as a standalone experience without any DLC plan. However, we have ideas on how we could extend the story of Basim, surely. But as of today, no post launch content is planned for Mirage."
So, even if there are irons in the fire, don't expect to see them come out any time soon. Currently, Assassin's Creed as a franchise is going off into completely different directions. We'll finally get to see Feudal Japan in Assassin's Creed Red and Assassin's Creed Codename Hexe will give us an experience unlike any other.