With the release of Total War: Warhammer and its upcoming follow-up, Total War franchise has expanded beyond historical games and new titles are still in the works. Therefore, in order to avoid ambiguity, Creative Assembly decided to introduce a new class of Total War games. In contrast to the main installments in the series (like Total War: Rome II or Total War: Shogun 2), A Total War Saga will be a spin-off series, or a separate category of historical standalone games that focus on a relatively short, but eventful period of time (similarly to what Fall of the Samurai did). Jack Lusted from Creative Assembly explained that on the official blog:
Total War Saga games will be the same mix of turn-based campaign strategy, real-time battle tactics and hundreds if not thousands of hours of gameplay as a regular Total War game, but focused down on a distinct moment.
Games labeled as Total War Saga will revisit settings already featured in major Total War games, but will present some specific historical events and single geographical areas instead of entire eras and continents. The first one is going to be a follow-up to Total War: Rome II, like Total War: Attila. The game should be announced in a few months.
There is also a major installment in the works, which is going to feature an era previously unexplored in the Total War series. That one, however, will come out a bit later.