By the power of truth, I, while living, have conquered the universe.
Not really sure what Bungie was trying to accomplish with Halo 3's ending.
On one hand, it was the end of the trilogy, and seemingly the end of John's story arc. However, they deliberately left about a dozen loose ends and had Mendicant Bias say that he was beng sent to the Forerunners. The only thing that Halo 3 resolved was the Covenant's role in the series, there are still many problems the UNSC have to face.