I think there will be a Halo 4, and even more books after the the next one, which is guaranteed. There's ALOT of ends left untied.
1. There's several Spartans still alive, and/or unaccounted for.
2. Bungie said Halo 3 would end this "story arc". I take "story arc" as the Human/Covenant war.
3. At the end of Halo 3, the planet. First off, see any greenery, ice? No, it appears metallic in nature, which the Forerunners were known to make every damn thing out of. Plus, there's obviously lights and the surrounding features of a city, which happens to seem circular. Everything Forerunner was based on circular designs. And last, it's outside the reach of the Halo's.
Another note, it CANNOT be Onyx. Onyx was made of sentinels, and the majority of it was blown apart by Kurt, most likely including the sentinel facility, thus is couldn't have been rebuilt.
[/end rant] lol Just my opinions.