I agree with Glen that this would have to be a separate site. Beyond which, I don't think PPCG is a good analogy, for reasons I'll elaborate on below. An Engineering puzzles and challenges site would have to operate by its own rules and get through its own growing pains in order to succeed. I like the idea that our community could have a relationship with such a site because I do like puzzles and so do many of our users.
Why doesn't Code Golf work as a proof-of-concept?
Code golfing became a "thing" on Stack Overflow (SO) way back in 2009, prior to the launch of Stack Exchange 2.0, and some users considered it disruptive even then. The trend was moved to Programmers SE for a while before it got its own site, but not without leaving behind some maintenance work.
The fact that there's an active beta site for Programming Puzzles & Code Golf (PPCG) has a lot to do with the overlap between SO and PPCG—in terms of domain, user base and technology. We are not an established resource in the engineering world; we've been around for less than three months. SO had been around for years already when the PPCG beta phase began.
The challenges on PPCG are also judged by objective criteria that are simple to assess using the same technology required to compete and access the site. Engineering puzzles—the interesting ones, at least—could be much, much more difficult to judge because prototyping and testing are much more involved in our typical problem domains than in the programming world.
Thinking about opportunities for expansion
We identified the challenge of accepting Naive Design Questions (NDQ) two months ago today. The fact that users keep bringing us NDQ is strong evidence that there's a desire to engage in some kind of collaborative design process, and we are seen as a potential outlet. That is exactly the sort of evidence that justifies turning a site into a network, launching a spin-off site or developing a parallel, off-site resource to expand what the site can do.
Those sorts of expansions have been on my mind since the NDQ discussion. From the very beginning, some of our users have wanted a different resource out of Engineering SE, one I'd argue we can't effectively provide on this platform. There are practical differences between programming Q&A and engineering Q&A, such as how much easier it is to evaluate and mark-up source code on this platform than, say, construction blueprints and mechanical schematics. Working through these differences and finding our way to a mature, established Engineering SE should be the primary focus of this beta.
At the same time, I would love for us to have an ongoing conversation about to what extent this platform enables us to collectively solve our engineering problems at work, at school and at home, and to what extent it limits us in doing the same. Problems based on naive designs or fun puzzles are still problems, after all. As much as our Goal with a capital G right now should be on working within those limitations, figuring out how to grow beyond those limitations could be a reasonable Big Hairy Audacious Goal.
What does this mean for Engineering Challenge Questions?
I think we could revisit this feature request in the future, when Engineering SE itself is more mature. I'd recommend holding off on an Area 51 proposal until there's strong evidence that users want this—personally, I'd want to see that we're regularly having to turn away quality content that could work for the proposal. (Of course, it's not up to me—anyone can start a proposal if they like.)
What we could talk about doing right now is something that has already been suggested in the chat room—namely, weekly topic challenges. While this doesn't involve engineering puzzles, it does challenge you to ask and answer at your best, and if done correctly can give a site a nice boost. If that sounds appealing, we can open another Meta question in which to start gathering ideas for challenge topics.