1. Make the Didact a dynamic character
2. Go to Sangehelios
3. Re introduce the Arbiter
4. Arbiter dies
5. Sarah Palmer dies
6. Halsey dies
7. Cortana doesn't return
8. Longer campaign
9. More "exciting" ending
10. Good story in general
Two reasons, first is that it isn't MJOLNIR armor, and second technology doesn't stay dormant, the ability to augment humans has improved.