Great article, only a handful of misinterpretations. I will chalk these up to you having not yet read Halo: Glasslands, because otherwise your article was great.
For anyone not wanting spoilers of Halo: Glasslands, and somewhat Halo: Cryptum you have been warned.
When you talk about a new ancient threat, you are correct in that the antagonist(s) will be "new" to the games, but probably not new to the universe. The likelihood is that the Precursor "race" will become far more involved this time around, and likely an enemy. Yes, Precursor, not Forerunner. Forerunners will likely make an appearance in some way, but I doubt they will be fully flushed out, at least not in the first game. I expect that the Forerunners will bee explained greatly, but that the Precursors, the equivalent of the Forerunners to the Forerunners, or some aspect of their time in the universe, will be the enemy. We learn in Halo: Cryptum that the Forerunners had captured and imprisoned what was believed to be the last precursor, and that he/she/it is very angry. Though thought of as deities to the Forerunners via their history, the evidence in Cryptum is to the contrary via this imprisonment.
That said, one could also argue that the Forerunners, if they have survived in the sphere this long, have split into factions, and that their is a Forerunner civil war raging on within the sphere. Forerunners could both be ally and enemy. Either way, expect involvement of both the Forerunners, and likely the Precursors, where new "races" may be introduced, but their overall existence falls within established cannon, at least distantly.
Another point to make is against the Covenant being buddy-buddy with humans. Glasslands shows readers the exact opposite, and that, although some are working towards peace, members of both sides are attempting to stage coops and finish the other off. Do not expect to see humans and elites fighting side-by-side. If anything, expect a race against a common enemy, while still fighting each other (similar to the flood). That is not to say that isolated UNSC/Covanant forces unknown to the war ending are not still fighting, but the explanation to this argument in your article is likely false.
Which brings me to the last point. When the warthog appears in Forza 4, it is indeed from the Halo 4 "make and model". This does not directly connect the UNSC to the battle in the sphere. John is on a UNSC ship, or at least half of it, and the vehicle hangar is his half. It is likely a Hog from his ship. It is unlikely that the UNSC is fighting already inside this sphere, because the plan for the sphere in Halo 4 is discovery, mystery and unknowns, much like you have written. If the UNSC is already there, if John meets up with them there won't be much mystery after his briefing, or at least a lot will be flushed out.
I greatly enjoyed the article, but only wanted to make some clarifications from Glasslands and some points derived from Cryptum.