I don't see this is as problem that needs address. I would find it very confusing if Alt-F4 did so many different things. I like that it just quits applications!
Ubuntu already offers some keyboard shortcut possibilities for "Logout", "Lock Screen" and "Suspend". Look under System: Preferences: Keyboard Shortcuts.
I now use "Ctrl-Alt-L" frequently for "Lock Screen", and that's handy.
I agree with this idea. Mark, what else does Alt+F4 do? Conceptually, the desktop is also an application, and closing it should offer to log you off. It's actually very instinctive.