What I want from DT's music? well, I'll tell you when I hear it!
Elaborating a bit more on the issue... music is an elusive thing. It's art, it's personal preference, it's something that has to touch you. There is no perfect formula, no scientific proof that grants that if you put certain elements in a song, it will turn out a song you love. One of my absolute favorite songs from them is The Mirror, but I'm feeling very meh about This Dying Soul for example.
More generally, I want from Dream Theater, well, Dream Theater's music, nitpicking aside, I must say that I appreciate the very vast majority of their entire body of work. One specific thing I could do without is the random solo section that breaks the flood and the mood of the song, that is the single thing I actually don't care at all for. Metropolis has a very memorable solo section, to make another example Trial of Tears has a long solo section that keeps you "into" the song, but on the later albums we have examples such as The Ministry of the Lost Souls, Endless Sacrifice and Sacrificed Sons when they're singing about a serious issue and then ask me to completely forget about it while I hear them doodling on their instruments for five minutes before going back to whatever was the mood of the song.
For the rest, I'm absolutely and completely in love with The Astonishing and I hope that this approach, translated into the common structure of a normal album rather than a double-disc rock opera, will be carried out with the next disc. A lot of emphasis on the melody and the freedom to explore different styles and weird song structures; I don't want to be able to listen to a two minutes ballad or an always changing epic with barely repeated parts only if they're on a concept album, I want them to come up with these odd songs in regular albums as well! but I'm willing to embrace with open ears and attitude whatever they will do next time; it's what I do since I became a fan in 1999 anyway.