They said they wanted an ever changing and evolving world, this is actually a really good way to do it. Every week there could be three events for 24 hours at a time such as you described. The outcome of the week or the previous event would determine what happens next. It could start on earth in the tower with mostly fallen and slowly mix hive into the battles. The more successful guardians are the more you could see the city change. While the first events would be defending the city from fallen, the following events could be pushing the fallen further back and fortifying the city. Eventually the events could move to the moon and then venus