One of the things that has helped Halo to stick around for so long is it's huge versatility. For example, the Halo 3 campaign was filled with tons of easter eggs and unlockables that could make playing through the game a second (or third) time still incredibly enjoyable, and it's multiplayer was always filled with a variety of hot-swappable game modes.
To top it all off, Forge gave players the ability to create their own maps and game modes, and it’s back on the Master Chief Collection, and it's better than ever. Whilst players will be able to play Forge on the Halo 2 maps, they will also be given access to three different blank canvas maps that allow players to create to their heart's content.
To make up for the lack of terrain on these empty levels, 343 have added a large range of natural scenery formations including flat pieces of ground, hills, trees and rocks, all of which can be used to create a nice base for a level.
A lot of new features have been added to the new Forge on the Master Chief Collection as well, including buttons that can be used to open chosen doors, spawn vehicles and Forge objects, and do many other cool things.
I expect the new button feature alone will open a lot of room for creativity. Halo The Master Chief Collection will release on the Xbox One on November 11th.