There should be a preference pane to enable and customize a set of screen corners and screen edges that close / maximize / minimize windows, close tabs, and switch to next window / tab, previous window / tab, next page in history (when the active window is a browser of some sort), previous page in history. All of these mouse gestures will operate on active windows, and will allow users using just the mouse to quickly and intuitively operate on windows. Something similar is already implemented in Compiz, which allows you to customize screen corners and edges which, when the mouse touch, user-defined keystrokes are executed. We can do the same thing but make the preference pane and selection of what commands to execute for which screen corner / edge more user-friendly.
While in Activities mode, there should be keyboard shortcuts that enable users to create a new workspace, switch to the next / previous or a specific workspace / window, or select a specific workspace / window and make it active or close it. There should also be a trash corner that users can drag application windows to in order to quickly close multiple applications.
Direct Selection of "active" tasks with key combos
The idea is to allow <special key>+<number> to "foreground" a particular window/task from a special list of windows/tasks. This special list can be created in three ways. First way: when <special key>+0 is pressed the most recent 9 windows/tasks from the "alt+tab" "window stack" are copied to that list, where "1" is the most recent, and "9" is the 9th most recent. Second way: when <special key>+<modifier key>+<number> is pressed the current foreground window/task is set to item# <number> in the list. Third way: if <special key>+<number> is pressed to raise the relevant window, then <special key>+<equals sign> is pressed, the previous raised window would be now be assigned to <special key>+<number>. This allows the user to see what the assignation is before choosing to overwrite it or not. If the previous raised window is the current window, the current window gets assigned when <special>+<equals> is pressed. If <special key>+<number> is pressed but has not had a window assigned to it, the current window is raised. Special key might be the "windows key" on PC keyboards (preferred default, alternatives could include the "app/menu key").
This feature would allow expert users to rapidly setup a shortlist of windows/apps/tabbedwindows and instantly switch amongst them without lifting their hands from the keyboard. Say you are debugging someone else's code, you could have 5 related windows (ssh, docs, logs, browser, etc) open and quickly switch amongst those. Then you need to work on something else, so you set up a new list of X related windows. While windows can be moved to virtual desktops, you can't switch to a particular window with just a key combo, and you can't currently move those windows to a desktop as quickly.
It is common for trained users to be able to operate application specific user interfaces (cash registers, data terminals and similar) very quickly and effectively (tabs, function keys and all). With this feature, many different windows could be used almost as quickly as a single integrated application.
This was also suggested to KDE Bug 121349 in 2006. It is still superior to what Win7 currently does with winkey+N (which is only helpful for nonexpert users who work with a few windows and tasks, and the sort of users who can't cope with more than 9 windows/applications open).
Something like this: https://sourceforge.net/projects/linkkey/ (but for Gnome and not windows)