I agree with Soli. I think the Devs, ultimately, have to put out something that comes as close to their vision as possible.
What I think Sub and some others are alluding to is that the people with the most play-time are a great resource in helping Devs arrive at their vision. I see nothing wrong with the Devs giving out generic design ideas such as, "We want to encourage teamwork and Class A's role should be one such as XYZ." And, if they are having a hard time coming up with a way to accomplish that, maybe ask the players what they think could help accomplish it.
Of course, as players, we tend to be biased.... but it will be up to the Devs to sift through those suggestions and find the ones that help accomplish their vision, as opposed to implementing things that create sweeping changes based on biased player feedback.
Cliff Notes: I think a mix of the two ideas is crucial to a successful mod (not that I would know... just basing it on theory). The Devs should have a vision, but they should certainly listen to the experts (I.E. Hardcore players with 100s of hours of play time) to help realize that vision... not necessarily change the vision.
Tones