There is no doubt that more Force Awakens mini's will come out 'eventually.' And yes, FFG says that Resistance can use Rebel stuff (and vice-versa) and the First Order can use Imperial stuff (and vice-versa).
But what's the consensus? Do we think that after more models are released the two "new factions" will become fleets on their own? Making the official fleet count go from 3 to 5? Or will Rebels and Resistance always be opposing Imperials and the First Order?