We're making an infinite sidescroller for our project. I just finished making all the obstacles and collectables spawn. The intervals for when they spawn and what y-value they spawn at are based on a base value and a random number within a set range being added to them. Also, the objects move faster the longer the game continues to increase the difficulty over time.
This is a jetpack joyride style game with two players trying to outlast each other. We plan to add powerups, but don't know what they will do yet.