Getting the Data Dance is easy to do when you get the animator to watch all the frames, herp.
Barrett, as Edich said, should be a friendly rival, something of a foil for Volnutt. Volnutt could take on some of Barrett's quirks as the game progresses, and vice versa for Barrett—excellent opportunity for character development, which is something this series has done well on thus far.
As for Purifiers: I wouldn't be surprised if there are some MS Purifier Units left—you could fight one and, post-battle, the defeated PU could retell Volnutt/Trigger his viewpoint of seeing Trigger ram through the System's own defenses, intent on shutting the whole joint down. This PU would have lost his own comrades to a former role model for the PUs—understandably, he'd be at a loss as to why Trigger would do such a thing. This kind of backstory development could work well with a few fights with some ES Purifiers, who never even knew Trigger.
I'd like to see a big Reaverbot raid on Teomo somewhere around the middle of the game, where parts of it could get leveled if you aren't quick enough to defeat the opposition. This would also provide a dilemma for the player, being forced to choose which parts of Teomo to preserve first.