Doc Eggman is completely right.
Single Person modding was able to work in the days of sprite based games, because it all involved 2d art basically, that and some minor coding.
But these days, the days of 3d, everything is much harder. It requires a team of dedicated people, 99% of the team being unpaid, to make a modification.
The team must consist of coders, mappers, modellers, animators, sprite artists, texture artists, 2d artists, and sound effects specialists.
Finding qualified, dedicated people in these areas is difficult, because they are not easy to learn how to do.
So modding Doom 3 would be possible, very possible actually, but it would be quite a bit harder to do, and impossible for 1 person.