I have a feeling The Didact will be the main antagonist until about halfway through the game, i think eventually he actually needs your help as the precursors have returned to finally finish off the forunners and that could be the main plot.
Another scenario could be that the Precursors have returned to finally give Humanity it’s trial for the mantle.
And finally, the didact has lost control of the planets promethan AI’s and somebody is controlling them, for example Faber or Medicant Bias
Maybe it’s something completely new, maybe it’s a new halo ring threat, maybe didact has corrupted and is the antagonist.
Discuss.