Well, I meant to keep it a secret, but I guess there's no real point to it, so I'm just going to go ahead and say it.
I've had far more time to work on Tortured Planet than I originally thought I would. As a result, v9.0 is going to have significantly more than just new music. In fact, every level is being meticulously polished, optimized, and tweaked. There aren't any new paths or similarly obvious changes, but there will be countless minor to moderate improvements just about anywhere you look.
Expect tons of eliminated bugs.
Expect a few design nuisances to be lessened or removed, (for instance, steep staircases that played havoc with the MFE camera have been widened).
Expect dramatic optimization (I've shaved a few MB off the size of the file with no real loss of content, but this may be offset by all the new OGG files).
Expect loads of texture alignment and texture changes.
Expect a bit of new scenery here and there.
Expect the Tortured Archives to play a little differently (the levels are optimized but otherwise still look and play exactly like the v1.0 versions, and when you complete the levels you will be sent back to AHZ instead of the title screen).
Expect lots of new minor secrets, such as hidden powerup monitors.
This is what I have done so far, and I intend to keep doing more until Charybdizs finishes the soundtrack. So the answer is yes, I have absolutely been working on Tortured Planet. v9.0 will be a fairly big leap even beyond the cool new music tracks.