More versatile Objects that are able to adapt easy together and still look good at the same time like Reach.
Everything in Reach came together so nicely and clean, where as in Halo 4 some objects look ugly with other objects.
The only thing bad about Reach forge was how poorly the Forunner designs in the forge pieces came out, giving it that plain and stale look.
Bring back Forunner objects or better yet bring in the Promethean designs they both look the same...But make the designs in the pieces show more than they did in Reach, thus giving them a contrast but still having that versatile feel to them how they had in Reach!
Example: All you have to do is make the lines/patterns in the pieces pop out in a more bold way. Make the lines darker so the pieces don't feel so plain
Bring in something like Sandtrap from Halo 3 where all the pieces where plain and simple and the only aesthetics needed was good forging