Maybe Their should be a ancient human who uses spartan like armor but instead is called a Trogan, he could be from a forgotten human planet unknown to the UNSC. He had a friend who was a scientist who found a cure for the flood infection but it was too late because the halo rings were already gonna be fired. So he sent his Trojan friend to the future so his cure could be remade and further mas produced. This Trojan should have the power of teleporting by using one of his armor abilities this power could help chief open doors from the inside. He has to find Halsey and give her the cure. He than realizes that he could save the future by going into the past and making sure the flood never appeared to cause the infection. The master chief realizes it could save humanity but it would prevent him from meeting any of his friends and cause some people to never be born or risk changing the future even further . The chiefs stops him and tells him to join him but, The Trogan decides to be the protector of time and make sure the flood never appear again.
-this could finally get rid of the flood once in for all and help move forward to the precursor story line
- this could be used in a halo 4.5