Just spotted a new release at the Doomworld forums from Dave Billing which has the promising title The Phobos Incursion. The project is a new episode 1 replacement for Doom which does exactly what you might expect: Deliver 9 new maps from scratch heavily inspired by Knee-Deep in the Dead. It catches the look and feel perfectly and also gameplay-wise is a very interesting experience you should definitely try. Check the release thread and the authors homepage for more information and also a download link.