Suggestions:
-Humans learning about or inferring the existence of Covenant technology (like Luminaries) and how their tactics have to change.
-Reaction of tertiary Spartan 2 Program personnel to the fact that they're training child soldiers. What do the guards and technicians think?

: Something different that I'd like to do, I'd love for this to either be
: chosen by someone other than myself, or randomly drawn (I might enter all
: the themes for a given week into a system that chooses one at random),

Sounds like a good plan.

: primarily because I want to participate as well, and I want to be
: creatively challenged with some of the subjects.

I'd love to read what you come up with. Your entry to the "Unknown ship from the comics" round was inspired.