Carmack next engine.. will not be a rewrite of Doom3 engine,just a major Update..in the Lighting and shadows.. very high quality softshadows and superior accuracy in the speculars and normal mapping.. means the end of the plastic look some people usually complain and the end of the pitch black shadows. he already states that when he finished Doom3 already new stuff was added , in the engine and that with the things he is working it will be able to match Pixar movies..
hopefully it will be an game with more outdoors and vehicles as default.
btw.. take a look at Doom3 graphics
here... the creativity of people when using custom textures...
its not finished yet ,work in progress ,but as you see you are not limited to Doom3 style graphics .you can design levels for any MOd you will like to see. seems they are looking to do an ALice and wonderland level . if you have skills and some imagination anything is possible .