In my opinion what Freedoom missed from the start of the project is any kind of creative direction for the project. This is something that is probably my fault as I was running the project from its inception until a few years ago. I focused too much on just getting as many resources added as possible to create a complete game. As I'm a programmer, not an artist, I focused too much on the technical challenges of the project and not enough on the importance of the artistic parts. The problem is that the result is a mishmash of different material and different styles.
There are many different ways I can imagine to design a creative direction for the project. The simplest would be to have a single person acting as creative director, designing how the project should look, but if you do that, you end up with a single point of failure, as that person has to maintain an active interest in the project.
There are other more passive ways that you could try to encourage a creative direction. The simplest would be simply to write a story for it, perhaps divide up the levels / episodes and describe how the game is supposed to progress. That then gives the level designers and artists something to work to. You could extend this by coming up with some mock screenshots, so that others can get an overall feel for what the game ought to look like when completed. For a design document like this, it probably ought to be a single person or a very small number of people doing it, but at least this way they are not necessarily a single point of failure for the project in the future.
The main problem of course is the sheer scale of the project - Freedoom still doesn't have a complete set of monsters, for example. It takes time and dedication to draw out all those sprites, and we've only found a few artists who have been able to contribute them. The same applies to the other material (levels, textures, etc.) although on a lesser scale.
What I would say is that I think the lack of creative direction for Freedoom is actually a deterrent for people that might be interested in contributing. I get the feeling that informally Freedoom is something of a joke due to its mishmash of material and isn't really taken very seriously. You might find that if you have a solid plan for what the game should be like, you'll be able to attract contributors more easily.
Finally, one last thing - don't underestimate the legal aspects. One thing I am glad of with Freedoom is that everything is freely licensed, and that means it can be included in the Debian repository (among others). That was identified as one of the key aims at the start of the project, as it was really the main purpose of Freedoom. Make absolutely sure that everything contributed is original work and actively make an effort to explain this, because there are people who don't care or understand about copyright issues. Unless you explain and make clear what is and is not acceptable, people will rip stuff off websites, other games, etc. Even if you do explain this, there will be people who don't bother to read your explanation, so be vigilant.