Idea 2: Draggable tabs everywhere.
It's not much of a stretch to expect Apple to bring draggable tabs to Leopard, and make them an integral part of the user interface. In looking around for inspiration, I found that the excellent chat client Adium already handles tabs almost exactly as I'd expect them to work. Apple just needs to make it easy (and expected) to have draggable tabs available everywhere.
Imagine every window spouting a tab in the lower left corner once you hovered your mouse pointer there for a moment; leave and it disappears again, but if you click it, it sticks around, complete with a red dot close box to get rid of it. Click and drag on the tab, and it behaves like a title bar. Drag the tab next to another window's tab (or where its tab would be), and the two tabs join windows, leaving one window with two tabs.
That's the simple genius of draggable tabs: it allows you to organize your windows together in logical sets. It also solves some real problems for Expose and the Dock in a simple, intuitive fashion.
Draggable tabs can be integrated into Expose's window functions, so when you F9 or F10, the tabs all split out into individual windows for Expose, and collapse back into your original tabbed sets once you leave Expose. Wow, suddenly the problem with Expose being worthless when used with Adium's or Safari's tabbed windows goes away! Expose becomes even more powerful in displaying and non-destructively navigating through open windows, a particularly important idea when you consider how tabbed windows are becoming more and more common; the next version of iChat is rumored to make use of tabs. Apple just needs to make draggable tabs (and split panes) a standard component on all windows.
Additionally, draggable tabs mean that the Dock is no longer exhausted by too many loose document icons. Users who routinely open twenty emails and minimize them all into the Dock can now group their open windows together into sets, using draggable tabs. Now they'll have just a few windows open (or in the Dock) instead.
Tabs belong everywhere. iChat, Terminal, TextEdit, Mail and even the Finder all deserve tabbed windows for their documents, so simply making tabbed windows part of the operating system, and making it easy for developers to add the feature to their applications, makes a lot of sense. Draggable tabs and split panes are natural partners. Another complementary feature is the idea of...
Idea 3: Stacks