I've always enjoyed making maps for games. I originally started with Far Cry, the Warcraft 3, some others, and now halo forge. I think it would be really cool if you could select mutiple objects at once. Say you build a really cool base, and then you realize it would look way better somewhere else on the map, or you just want to get it in the right spot. rather than editing the coordinates for every piece You should be able to select all the objects and move them all at once... it would make it alot easier. Anyone who's made maps in FarCry probably knows what I'm talking about.
This is one of the things people really wanted as an aspect of forge, but grouping objects to move them can take significantly more processing power than moving individual objects, and we have to remember that Halo is first and foremost about shooting, not forging. There's a chance it will be added in a future version of Halo, but we have no idea what direction 343i will go with Halo 4.
In addition to group select -> move, it would be great to also have group cloning (ex make a house from 10 pieces, clone that build to make a row of houses), as well as group mirroring (ex building a space ship, you build one half and then mirror your half to make a perfectly symmetrical mirror copy). If they could create a group move tool it would have to have it's own unique local position of the group, so when you rotate all the objects together they pivot off a central point, and not at their individual local axis. What would be REALLY cool is if you could use this group move tool for some kind of geometry fusable cockpit. So that object could be "glued" to a mover and driven by a pilot. Then you could actually create moving giant spaceship and boats!