Nice idea, but I think the idea of requesting specific models doesn't play too well with sites offering free, unsolicited material. The reason why there are maybe 201 variants of a Murcielago on the 3DW is probably because they are all different mods of the same ripped-off commercial model by Dosch, or whatever....mostly done by teenagers who couldn't produce an original model to order if their life depended on it.
There's obvious advantages in the model challenges, as Pete points out, but I find it hard to imagine a dedicated bunch of specialists churning out free model requests just for the kudos. A request means working to a brief; and most of us have enough of that at work...we prefer to fly our own kite in our free time.
FormFonts will model to order...you can request a specific item of furniture, or species of tree, or person engaged in a particular activity. Up to a point you get it "free" as part of the subscription, but that's the point; it will cost you the price of a subscription. People here at SCF do occasionally oblige with a plea for a specific model, but I don't see it ever becoming more than an ad hoc thing...that would smack too much of the work treadmill.