It seems like Savathun will be the final Hive antagonist from the sounds of it, as she seems to have the most control over the Hive (implied even as far back as D1) so far as well as the Taken. Taking her out would likely put an end to both the Taken and the Hive. I also hope that we eventually get to see the inner circle of the Vex Collective Mind; the closest we have gotten to is Atheon and Panoptes (not sure if the Dark Heart counts since it can be reconstructed).