This is an educated perspective, and gives a potential deeper meaning to the initial reveal trailer of the new title. If that giant bot is a guardian, then the question then becomes what exactly is he guarding that the chief finds so important?

The showcasing of Cortana's chip could signify his motivation to fighting whatever needs to be fought, regardless of who the enemy is. Though, I don't know how my Battle Rifle would kill such a large enemy.

We cannot forget the ending of Spartan Ops. We now have a key that gives us the location of all of the big toys of the Forerunner. It's now easy to find particular weapons, vehicles, and other technological items that can help humanity, and Master Chief's goals. Though, some of that stuff needs to be guarded; great room for a potential new Forerunner enemy.