I would love an update that focused on improving custom game options. Here are some suggestions I have:
1. Add more movement options for Halo CE and Halo 2 Classic. Basically just give us the options that later Halo titles have, such as increased movement speed, jump height, etc.
2. Allow us to start with any weapon. In Halo CE, you're limited to certain presets, and in Halo 2C and 3 certain weapons aren't available to start with. Three examples are the Sentinel Beam, Fuel Rod and Beam Rifle but I'm sure there's others. Being able to start with any weapons could open up more custom game options.
3. Allow skulls in all games. Giving us the option for skulls in Halo 2 Classic was a great start, but if we could get those skulls in Halo 3, 2A, and 4 the Forge possibilities would be endless and allow for things not initially possible in the original releases. Also allow us to use the scarab skull in customs please lol.
I'm sure there's other things that could be added but all of these would be amazing.
Edit: Also, when Reach comes out I would like the option to switch between the different settings of the game in custom games (Launch settings, Title Update, Zero Bloom, and Anniversary). The original Reach didn't have any easy way to switch between them in customs.