I think that when it comes to mass-production of a finalized design, its still going to be hard to beat injection-molded plastics. That being said, it takes time and money to make the molds, and that's an expense that has to be amortized over the production run. Glock introduced a plastics/metal pistol in 1982 to mixed reviews, but its now clearly a HUGE success, and every major pistol manufacturer offers a model where a major portion of the complex frame is non-metal.
This machine is awesome for one-offs, and low-production designs, which fits right in with E-bikes right now. I'm working on an idea for a disc-side hub-motor fan similar to a front disc brake from a car. I plan on using straight vanes that are angled, but curved blades would work better (harder to make for me, easy in 3D).
Another project could be a a mid-powered mid-drive bracket. With the right shape, thickness, and ribbing, 3D plastics could easily make something that could work well. The drop-outs could be water-jetted metal that bolt-on. Since I would imagine such a bracket would be low-production, you could just print and ship, according to what frame it was for.
I wouldn't want to step on the toes of urbancommuterstore.com or StokeMonkey, but...Rolls-Royce was introduced in 1906, and the Ford Model-T in 1908. Both are still with us, but one of them sold 15-million units. The world needs a Model-T of E-bikes.
Triangle battery/cargo housings, perfect product for this. Low production and the shape varies by frame. I'd make it in three corner pieces. Made so that the end user could buy and cut their own side plates out of aluminum or 1/4-inch plywood, the side plates would hold the whole thing in alignment. Of course if they are popular and the customers are willing to pay enough, you could sell the side plates too, but at least the customer has the option. Tubing clamps could attach from the side, and be integral to the housing...and the clamps would match the odd shaped tubing that's sometimes found (triangularish or oval).
edit: if someone actually wanted to make a triangle box as I have suggested, make it as big as the biggest frame candidate, the corner pieces can be cut down smaller to fit a smaller frame.
edit: I'd add two simple hinges on the right side so that the right-side plate can swing out easily for access, or be cut-off by the buyer if they wanted that.