SUB MULTI-TRACKS (MULTI-TRACKS INSIDE MULTI-TRACKS)
You could add the awesome option of including multi-tracks inside other multi-tracks parametrically, such as After Effects does with its pre-compositions inside compositions ;)
Olaf Nelson commented
This would be the biggest improvement possible. It would free up memory/CPU by making it possible to work only on one part of a complex project without having to load everything.
It would make it possible to use one session in multiple other sessions without having to duplicate the work or do lots of copying and pasting, and if changes were made to that one session they'd happen automatically in all the projects where it's used.