Unfortunately, this is my opinion, that Bungie should stop the Halo campaign and single player series.  You can't have it continue forever guys,  Remember when we thought Halo 2 was supposed to finish the fight?  We were disappointed but looked foward to Halo 3.  Now that the ending to Halo 3 is convincing enough so that the fight is finished, Bungie can focus on other projects.  Bungie is a game developer, they just can't focus around one game, I'm sure there are employees wanting to develop something other than Halo.

As for the Halo Multiplayer, I do believe that Bungie will continue to make maps, game varients, and maybe some new weapons and vehicles to satisfy the hardcore gamers.