It seems like Assassin's Creed Mirage will be a complete package come launch. While recent games in the franchise have been expanded with pos-launch DLCs and expansions, this will seemingly not be the case for Mirage.
As for how we know this, speaking in a recent AMA thread, creative director Stephane Boudon replied to a question regarding post-launch updates, stating:
"For now, Mirage has no plan for DLC or extensive post-launch."
It should be said that there have been occasions where a game has debuted well and caused developers to rethink post-launch support, something we are seemingly seeing currently with Final Fantasy XVI.
Would you like to see Assassin's Creed Mirage get a DLC or two?