Forge was a huge revelation when it was introduced in Halo 3. The mode became of favorite of not just level creators, but also those who just wanted to mess around with the physics of the game.
The mode will be returning in Halo 5 too. This new video shows the work that has been done, in what 343 is saying was built from the ground up. It adds a few key features two, maybe most importantly, 'grouping' which lets you tie a few object together and move them as if they were one. Take a look in this new video:
The mode won't be there at launch mind. It is touted as coming in December, so at least you will have some time to dream about what you'll be able to build.