Vertically split window
DISCLAIMER: This is intended to be an OPTIONAL feature.
It would be great to be able to bring the tool window, sequencer, and rack into a combined vertically split window, where the split points can be dragged left and right, but "snap" to the extremes. So for example you can hide the tool window (now a "pane") and use that space for the rack, or hide the sequencer and let the rack take up the full window. Or you can claim the rack's space for the sequencer, or have it take up the full window. This would be ideal for maximizing widescreen displays.
Only caveat is that you can't really have the big fat graphical navigators in this mode, there wouldn't be enough room. They would have to revert to plain old scrollbars.