I'd like to see the band try something different with their sound on DT15 but I doubt that will happen.
Yes, I know The Astonishing was 'different' but the sound of the band has remained unchanged since Portnoy left, even on TA. It was a "Disney-fied/Broadway-inspired" version of what the band sounds like in the 2010s.
Every album from WDADU to ADTOE there was an evolution in the band's overall sound, in one way or another, even if sometimes the band claimed to be doing a "back to roots" thing every few albums.
With DoT, that was another attempt at "back to roots" but I don't hear it, unless back to roots is "back to ADTOE." The sound of the band remains the same 4 albums post-Portnoy, all the band did on DoT was cut the fat compositionally compared to ADTOE, but they did that on DT12, sans IT; and DoT present a more edgy production and give the drums a boost, which is a positive, but long over-due.
Say what you want, and it was controversial when it came out, but Train of Thought was something radically different, though preceded with The Glass Prison, maybe Home, and a couple of other earlier songs. The raw heaviness was genuine, unlike the "look at our well-made metal music" trend that's been going on throughout their albums since basically Systematic Chaos.