Well Oryx has 2 sisters left, and they each have armies. Plus there's Nokris that we know nothing about beyond him being mentioned in a tablet alongside of crota, implying that he's some kind of God like them. We've got a cabal empire to deal with. There is almost certainly going to be am actual kell of Kells that we have to deal with. Variks is using his position and control over reef crows to search for the real kell of kells. Now, whether we will actually fight him or not is uncertain. The kell of Kells is supposed to unite the fallen. But if Variks has some influence, we very well may get fallen allies out of the kell of kells. Aside from their tendency to murder and steal, the fallen are probably the most relatable species. Get them in good graces with the traveller, their great machine, and we might be able to convince them to stop murdering and stealing from us and to just do it to other bad guys. And we still have a lot about the vex we don't know, such as their origins and their apparent inevitable downfall they're trying to prevent. And the ahamkara are almost certain to become a problem. Assuming the hive worms and ahamkara are the same (if they're not, then we'll have to deal with the worms too), they're clearly allied with the darkness and try to corrupt species with gifts. And they don't die easily, since they exist after you kill their bodies. And finally, the darkness itself. Not sure if it's a thing we can fight.