Currently, many users do not use activities or virtual desktop, because on some systems it are hardly accessible. So integrate switcher (ALT+TAB) of these ideas. Simply: when user click ALT+TAB, there would be show activity switcher and show desktop switcher items alongside windows thumbnails.
User simply click ALT+TAB, window switcher appears, press ALT+TAB until switch activity is not marked. Now, user release keyboard buttons and activity switcher appears. First position will be nothing , but second will be go up, so again window switcher appear, but user could switch activity pressing alt+tab and switch activity. Similar allow to switch virtual desktop. And: when selected another activity/virtual desktop, show window switcher again, but user could press esc to exit.