1. Customization is the key here... I'm sure alot of people agree with me when I say that I want a TRUCKLOAD of customization options. Everything needs to be customizable: Knee pads, Elbow pads, Wrist, Helmet, Visor, Chest, Shoulders, etc. I love having the freedom to mould my character how I please. Something along the lines of weapon customization would be nice, and in saying that, I know that alot of people will rage at me because weapon customization is used in CoD, but it's not exactly copying, even though they've copied a bazillion things from our series.
2. The credit system - I love it, it works well, it's fun to work for credits to unlock armour.
3. A.I. in forge? I know it's a hard thing to accomplish, but it would be great to have maps with AI's in them.
4. Interact-able forge objects? Things like open-able/closable doors, traps, etc. are fun. Kinda like when you make a noob trap on Halo 3 - The joy of watching your friend run for a power-up, only to get splattered by a "half-tonne dumpster" is great, but if there was a trap that could be activated by pressing an in-game button, it'd be even better.
5. Easter eggs. Oh god, please add some funny easter eggs. They were what made Halo 1, 2, 3, Reach so damn fun. The hidden grunts, microsoft sam, monkeys, etc. were great. Perhaps leaning more towards Halo 1-3's easter eggs, because of their hilarity.
I feel as if I put too much effort into something that will only be read by one or two people... I guess it doesn't matter though, if a 343 employee happens to be browsing Bungie.net (for some reason) and happens to stumble upon this thread, and HAPPENS to read my post, perhaps they agree with one of my suggestions and will implement it into Halo 4