More challenging you say? Hm. The game is about seeing what would come up next and react accordingly. So you may introduce something that would make it harder for player to notice such things (a fog that covers right side of the screen, so there's less time to react or a night level with thunderstorm illuminating the level once in a while, so that you have to memorise what's next, since you cannot see that all the time). I don't know, you're clever people, I'm sure you'll figure something out given more time than 48h :D Cheers!