Add another level. When the player wins, broadcast a level2 message and change the speed that the sprites move at, using a speed variable.
Make platforms appear and disappear in a forever block using show, hide, and wait.
Add more platforms, or make more platforms appear when you get to the next level. You will need to consider what else needs to change, to make your game work on the second level, and reset correctly to the first level when flag clicked.
Design your own top-down character in the Paint editor.