I stumbled upon this forum today and I couldn't be happier. I have a place to express my thoughts, hopes, and wishes about this game. Anyways, I remember reading the Halo Wiki and after jumping through 30 links (you could get lost in there) I discovered many things about Forerunners. Also, possibly more fightable enemies that Halo has combined altogether.
Lets start with what was supposed to be in Halo 3. In Halo 3, Instead of fighting many Scarabs, Bungie was going to release a new boss in replacement of one of the Scarab battles. Ladies and gentlemen, the Guardian Sentinel. You were supposed to climb it and disable it's eye, using the eye as a weapon. But it was sadly scrapped. This is also the guardians the game refers to when you die from environmental disasters.
If you scroll down even more on the page, you can see a chart of all Forerunner buildings, weapons, and enemies. It sure kept me busy for a couple of hours.