offline mode suggestion

Apart from the achievements, offline does not really have a great link to the online game.
so my suggestion is to have a boss level for each world on offline mode, and for doing a boss level you have a chance of recieving a bronze, gold or crystal chest or a gold or crystal key.
next time you log into online you recieve your award through attachments in your mail !
this would be a great addition to offline mode and would give a good link between both offline and online modes
hope you like my idea and leave feedback
Isaac