I don't know ... sometimes, I feel like things with Halo are made up game after game. I think they're now possibly doing a better job at this -- I think I vaguely remember an employee from 343 pre-Halo 4 time saying that they have the major story planned, just not in complete details for, say, Halo 6.

I really hope so. It can be obvious sometimes when the overall story hasn't been fully planned out. Imagine if Halo 1-4 was planned from the beginning, and we were able to go back to Halo 1, replay it, and found little hidden things that were given more meaning and depth. For example, what if in Halo 4 we learned more meanings of Forerunner glyphs, then were able to decipher a message in Halo 1, which lead to a huge breakthrough in knowledge of something big?

The little things.