I believe that the big finale of TFS will be that every enemy race that we have encountered *except the Taken and the Vex* will become our allies to some degree, the reason I think this is because we already have Cabal, and the Eliksni. I think that Savathun will come back as our ally since she is technically a guardian. And this will all result in the final battle of the light and darkness saga where Humanity, The Awoken, The Hive, The Eliksni, and The Cabal will all come together and wage war against the black fleet, kind of like how all of the MCU hero’s joined together to fight Thanos