Hi there, I love the game so much I just run through everything in 1,5 days and ended up at the Crucible.
My suggestion to keep the game alive longer would be to add something more than just the Crucible for the players. Maybe even before the Multiplayer patch, to add some dungeons or Rifts or whatever the name will be, where player could get increased difficulty and rewards forever - or until the next content update.
I sincerely hope there will be at least 5 chapters when its done, the game is such a fresh breeze and so fun to play, I just dont want it to have an end !