Would he be a secondary antagonist? No. He doesn't want humanity to have the Mantle, but he also thinks only the Forerunners deserve the Mantle. If he were to be in Halo 6, he would likely be working on his own. Of course, his goals may intersect with either humanity or the Created. Personally, I don't think he'll be in Halo 6. It's the close of the trilogy, and having him back would make it needlessly complicated. I am prepared to be proven wrong though.