This is based upon a game I made a couple of years ago whilst refreshing myself on C++. However this was made in one evening using GameMaker HTML5.
The player must collect orbs, which slowly shrink before disappearing. If all the orbs disappear it is game over but if you collect one then more are spawned. To provide an extra bit of challenge the player must avoid enemies that are either static, mindless drones or stalking the player.
This prototype only has a limited number of enemy designs and no bonus pickups or music, plus the drones tend to get a little lost, but you should be able to get an idea of how the game will work.