So for the past few months a have been looking at all the possible and impossible things the ending plant could be. I’ve seen everything from time travel to it being reach, but none over those make sense....I think its Onyx II
To start it off, Onyx was a Shield World created by the Forerunners as a shelter from Halo fire. Seeing as how Forerunners had the technology to built such megastructures I believe they could have been used as recreational use as well. The terminals possibly hint that the Forerunners built and lived on the Shields Worlds prior to them being used as a shelter.
Now the outside of Onyx was destroyed when Kurt uses the 2 FENRIS nuclear warheads to stop the Covenant from getting inside the "true" Onyx, the Dyson sphere in the center of the planet. Though now to the outside world "Onyx" is only a small sphere about 2-7 feet across covered by trillions of sentinels, on the inside it is about as large as the space between the earth and the sun.
Onyx takes place during the second battle of Earth, during this time the Ark is building installation 4 II, now from what we know the Ark can rebuild Halo Installations but is that all it's limited to? I leaning towards it was possible that the "anomaly" was built at the Ark then somehow transported to god-knows-where. The only problem with that is the timeline because the Ark was already in the process of building halo when onyx was destroyed.
So is it possible that there is another Installation capable of reconstructing shield worlds? I doubt the Forerunners wouldn't make an "Ark" of onyx in case for some reason Shield Worlds were being subsequently destroyed. The current planet looks relatively new also, with no "cover" of soil or water a typical installation would have, either. It somewhat helps me out because the new halo didn't have a finished structure ether other than the snow.
From what is in the halo encyclopaedia after halo fired the forerunners 'reseeded" the galaxy and left the Orion arm. So they could have ether built a shield world of a non scalable scale else where in the galaxy and planned to live there. Ether way I'm dead set that the ending is a Shield World recreated. Could it be the one from Halo Wars? It would fix the timeline gap and explain why the Ark's foundry is more depleted after 1 halo.
This is just the first post, now that I have the idea it could be the Shield World from Halo Wars I’ll make a second post when I have the time, now discuss please :).