I don't see Dream Theater going "full Djent", as they have never really embraced any other gente to their music through the years. They are the progressive metal band, that means they can go many directions without "shifting" genres, and that's what I think is gonna happen with the 8 strings guitar.
That being said, I'd expect something similar to the modern usage of 8 strings, like Animals as Leaders and Periphery use them, since those are bands that JP frequently talks about. Of course, I imagine he is also gonna bring something new and unique.
But to those of you who are worried and/or don't like the bands I mentioned, you should really consider being a little bit flexible when hearing what is gonna come from DT using 8 strings, in my opinion.