Fat Kid, Duck Hunt, Jenga and much more were memorable in Halo 3. I personally think 343 should add some custom game map remakes in forge or wherever. What does the community think of this?
Those gametypes are still able to be created in Halo 4, it just takes a bit more effort than previous games. Consult this thread for details on how to make Jenga and Duck Hunt.
Or, perhaps, are you proposing some sort of custom games browser?