Going to be honest, Forge pales in comparison to the old school HEK, especially in so far as that OpenSauce lets you make custom campaign maps and you can create entirely new characters, weapons, vehicles and so forth. It's great for messing around and making some basic stuff, but ideally when making maps I want some real power ala Source's Hammer or Gamebryo's Creation Kit.
It's a start, but all I really want is to be able to play through the entire Bungie-series of Halo games on the PC. I'm really not into Halo 4/5 at all.