As a mostly-outsider to the Doom community, my perspective is that new blood would bring both a wider range of interesting things and re-invigorate the work of (possibly jaded) long-timers. Doom predates the modern "indie games" movement by more than a decade, but in many ways they are now carrying the torch in terms of both originality and execution. See the FreeIndieGam.es
blog: every day multiple new games are posted; some are just building on old ideas, others are daringly experimental. This is what keeps a scene exciting.
I think with a few improvements Doom could fill an important niche for indie game creators. With 2.5D level design and sprite-based objects and characters, it's extremely easy to make content for. Unity3D is a great tool for many things but I've found its ability to quickly build interesting level designs is lacking compared to Doom's.
Unity3D's advantage over Doom is that it's extremely easy to define complex object behaviors in a level-agnostic way. ACS scripting is powerful but only within the domain of level scripting (ie instantial, special-case), while DECORATE is more about specifying behaviors via data which has obvious limitations.
That's why I'm so keen to see the ZDoom scripting branch progress; I think being able to define Thing behaviors in a general scripting sort of way cracks the possibilities of game creation wide open. The current difficulty of doing comparable things is one reason I think Doom engine projects take a long time. Unity3D by contrast is regularly used for game jams, it's ridiculously fast to get an idea up and running in a 3D space using easily reusable components.
Please undersatnd my comparison, I'm not saying that ZDoom et al need to become like Unity - their differences are in fact the reason I'm making this argument. I'm just realizing that the connections between these two very different technologies suggest that Doom could become much more than what it is today.
There might be some traditionalists who are ambivalent or even threatened by the possibility of lots of new people making things with Doom, but this is unfounded. New blood almost always revitalizes a scene without doing away with what made older eras great.