Just read over at Rex's Nexus that he is about half-way through finishing his latest project Deimos: Slight Return. It is actually in similar vein to his first release of that type, Phobos Revisited, building an episode that resembles the original and is 100% vanilla-DooM compatible, but has more detail, has tougher gameplay, and has extra areas to explore. So far the screenshots are very promising and a lot of work has already been done. If you want to take a look, just visit the project's screenshot page and maybe also give Rex a bit of feedback, I am sure it helps to keep up his motivation and make him finish the new episode soon (that's we all want, don't we? ;))