With the exception of Malta, AoE3 so far always released civs that met precise standards in order to maintain a coherence in the roster.
I’ve detailed here and there (forum) these requirements implied by the game needed for a civ to be considered as a candidate: they must first of all meet the game’s time frame (roughly 1450s-1890s), have produced an organized, sovereign, stable, influential entity, over a lasting span, and be somewhat related more or less to the Age of Discovery (although this last argument is on the verge of being defunct).
As such, a civ doesn’t compulsorily have to be an Empire: the US and the Mexicans were Republics, the Haudenosaunee were a Confederacy, that match all the above criterions.
With the Maltese, the lines were blurred, as Malta chiefly wasn’t even a sovereign nation. Nonetheless, as many have pointed out, Malta’s addition seems to be rather a nod to AoE3 in itself, answering an old request too. The civ’s extensive reuse of campaign elements, the HC customization with all the protagonists, the Morgan Black references as a politician and a card are all strong arguments supporting this view.
In conclusion, I think from now on only 2 arguments are absolute for a new civ: the time frame and the sovereignty.
Furthermore, let’s not forget the civ categories have to be somewhat balanced too: with 11 European civs now, one could reasonably expect the Africans, Asians or Americans welcoming some additions to equalize the balance.
Best,