Humans = Forerunner appeared not to be literal. Ancient humans existed 100,000 years ago, period. We weren't some space-faring conquering race. We just happened to be the place that the Librarian liked and the species that fascinated her. There may have been something done during re-seeding that allowed us to reclaim the Forerunner Mantle from the depths. It's almost like we inherited something. There are many, many places you can go with this story and with gameplay, especially with the inclusion of the evil Didact that could provide the Didact far more depth, not to mention a chance to really knock the human story out of the park (either showing human success or another failure at the Mantle, give or take a select heroic few like the Chief who live up to it or represent what's best in us all, blah blah trope-y stuff).