(This is mostly exaggeration)
The structure/Planet, seen in the trailer is a forerunner facility. And as some of you know. Humans are the Descendants of the Forerunners (Which is why we are able to interact with their technology). My guess weapon wise we may see some ancient technology included in the arsenal. But if 343i Decide to include weapon customisations (And/or hopefully an ADS system), a way to upgrade/modify weapons that we can currently use. If so it would add new and interesting ways to how the game plays (But at the same time defeating the whole classic Halo gameplay).
What i want most is the return of the flood. I haven’t fought them since 2007 and it’s become mighty annoying i can’t fight them anymore. I want to see the Flood at an even gorier/scarier level. iwant them to jump out at me. To have bloodcurdling screams, and -Yoink!- throttles. Definately want new/more variations of the flood if they are the primary enemy in the game. (forerunners are extinct.).