I wanna see the prisoner from charrum hakkor be the main antagonist. This would be tricky though because there are still 2 more books in the forerunner saga and we dont know what happens to him making it difficult to fit him in there.
Fight the Precursurs!!! the prionser from charum hakkor has raised an army and is set on destorying the reclaimers (humans). How cool would that be?!?!? also the forerunners emerge from the shield worlds to assist humanity. that would be awesome