Normally all the game assets and logic necessary were already downloaded to the device during the last apk update (basically plan ahead for daily/weekly events). You can also use an AJAX request to a server to get sort of a "message of the day", which can contain information regarding availability of certain functions or unlocks, and make use of load from URL for dynamic retrieval of sprites.
Like in a procedurally designed game, it is possible to describe how the game should be presented via data and variables, rather than placing and designing everything statically in the editor. Once your game an be built on variable data, it is pretty simple to use AJAX to get the dynamic content.