Halo 3 was the best halo game in every aspect of its sandbox, and every halo that launched after it failed because of modifying unnecessarily those aspects rather than adding new stuff to complement it.
Perhaps the only things it's inferior to other halo games is the lack of forerunner missions, the nonexistant brute plasma rifle, the magnum and plasma pistol being bad AD and the Elites design, which halo 2 nailed (h2a is great, but man, look at those dinos)
Other than that, new stuff from ODST and later like firefight, invasion, forge stuff and some of the newer weapons would have been great for H3, even more than in the games where those features first appeared.
