The original Edge of Tomorrow ended with a fairly definitive conclusion, so going back in time may simply be the only logical way to progress the story in a meaningful way.
Edge of Tomorrow introduced an incredibly bizarre alien threat to Earth in the form of an extraterrestrial hive mind with the ability to manipulate time.
Given what we already know about the Edge of Tomorrow universe, it seems that Doug Liman may double down on the time-bending qualities of the world established in the first film.
As long as we get more of Emily Blunt as the Full Metal Bitch we will be happy.