Just to give everyone a clear understanding, hitting the threshold on CUUSOO does not mean a set is being made, yet. It means Lego is willing to take a look at it, and if everything checks out will consider making a special kit of it.
This will be the 8th suggested Lego set to have reached the Threshold on CUUSOO (with an 9th looming). of them here is the breakdown. (and keep in mind, of those 9, all but the first 4 have hit the threshold within the past few days, and so are just entering the review stage)
1. Shinkai deep water exploration model, a real world submarine. Lego produced a limited run kit that released in Japan.
2. Hayabusa, the Japanese deep space probe. A real world real space subject. Lego produced a limited run kit, mainly released in Japan, but with some worldwide release.
3. Minecraft, this one we all know about here. Lego produced a special run of a Lego Minecraft set that will see some fairly wide release. Pre orders are now available. This was done through arrangements with Notch who was extremely flattered by the whole thing.
4. Sean of the Dead, the Winchester, would have been a truly beautiful adult "modular" set. Unfortunately after review Lego determined that the R rated nature of the source material was not appropaiate for their normal set guidelines. They will not be producing this kit. (at least not officially, although word is they all loved it. They just couldn't release it as a Lego product)
5. Eve Online "Rifter", This one just passed the threshold of 10k and is up for review. It will be interesting to see how far it gets. It is a licensed subject, but I can't see an Eve ship being a hard or expensive one to get. So the question will be is it viable as a product. To close to call whether Lego will make one.
6. Back to the Future sets, Delorean Time Machines from 1980's, 1950's, 1880,s and future. This one just passed the threshold last week and is up for review. The biggest hurdle it faces will be the licensing. I am sure this is a series of sets that Lego would love to make if they can get the license at an affordable rate. If the studio wants too much it will not happen.
7. Legend of Zelda sets, this idea also just crossed 10k. My suspicion is this is never ever gonna happen. Once again Licensing is king, and Nintendo is horrid about licenses. At best we may see one small special limited release set with a few figs and a backdrop, but I wouldn't count on that. Add to this that Lego's competitors have some dealings and licenses already with Nintendo for Mariokart and yeah, this ain't gonna happen.
8. Firefly, SIGH! I would love to see this set made, but my gut feeling is it ain't gonna happen. A combination of three factors. It is a licensed property, from a license holder that has a history of rabidly protecting it yet not doing anything with it. So a high or expensive hurdle there, problem 2 is the nature of the source material or its content. While not technically R rated, it does tend to dance on the more adult side of PG13. While not a show stopper in and of itself, it does put a damper on things when you factor in licensing costs, and finally we come to the elephant in the room, the age and limited nature of the property. 13 episodes, 10 years out and a less than successful movie. This is not a marketing guys dream project. Overall I think it adds up to a very slim chance of this being made.
9. Modular western town, this one has not yet crossed the threshold for review, but hopefully will this week. http://lego.cuusoo.com/ideas/view/1787 It's a great example of the sort of thing that Lego WILL make into a kit. No outside license, wholly creator owned design, complex adult appeal but no true adult content or nature. This one will go all the way to store shelves.
So the short answer for those not wanting to read, I'm betting the Firefly set is not going to happen.