In the Legendary ending of Halo 3, they showed the Master Chief's half of the Forward Unto Dawn headed towards a Forerunner planet. Any future Halo games involving the Master Chief might involve a Forerunner world, and hence would probably a LOT different from the earlier Halo games. I would imagine, however, that it wouldn't be called Halo simply because of the lack of Halos.
Not saying I approve (or disapprove) of the games going in that direction, but that's likely what they'd do.