Halo 4 is getting a new Forge map, with plenty of blank, open canvas for Halo level makers to paint their creativity on. As you can see in the quick video on IGN's page, "Forge Island" actually has three different islands of varying size, all completely flat and round, to give map makers lots of room to build whatever they want.
The map will also come with some new Forge pieces, which the developers have already used to build a copy of Hang 'Em High. 343 specifically asked at the panel if a fan would be kind enough to remake Blood Gulch on the biggest island, so that's possible too. The new map and related tools are all being released for free on April 11, just a few days after the Castle Map Pack arrives on April 8.