I would love to see support for multiple projects in a single window.
Personally I find the one-project-per-window approach difficult to use. Especially when I am working on two related projects, having access to multiple projects' files in one consolidated list would be very helpful.
How I imagine this working is splitting the sidebar up into collapsible top-level project groups:
Foo Project 1 OPEN FILES (Foo Project 1's open files list) FOLDERS (Foo Project 1's folders) Foo Project 2 OPEN FILES (Foo Project 2's open files list) FOLDERS (Foo Project 2's folders) etc.
Clicking on an open file or folder would switch the tabs shown in the current window to the corresponding project's open tabs.
With drag and drop support in the sidebar it would be easy to move open tabs and files between projects.
This method could also help with ST2's problem of adding random unrelated/one-off files to the project that happens to be open/focused. By optionally having a 'Loose Files' project in the sidebar, opened files that are outside of any open project's folder scope could be automatically (or manually) put there without opening another ST2 window.
Allowing both multiple windows and multiple projects per window (and remembering these between sessions) could work well for certain styles of workflow as well. Some 'search in all this window's projects' function akin to (or replacing) Ctrl+P would also be dandy.