Changing settings, changing tools. Hell, they could have made a modern one playing as Lucy or Desmond and interwoven it with Watch Dogs.
But people liked the first 3/4 games for the story and the movement. The Ezio trilogy is pretty much the same game three times! Make good stories and keep/improve on the core mechanics and they would have been successful.
Sponsors pay much more than views. So does patrons.
The true issue is discoverability in my opinion.