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).
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<The problem is that the only hints of Xivu Arath is that she's another sister of Oryx and possibly controlling the Cabal through Umun'Arath. Meanwhile Savathun's had a metric shit ton of hints and buildup. It's no wonder why people thought she'd be the main antagonist of DLC3. The Heart is not Vex. It wouldn't be part of the overall Collective. But it would be nice to see more powerful Vex.>