Backstory: 3d printing is becoming a viable product solution with recent product improvements (most noteworthy: Formlabs Form 2). We've tried to outsource a few 3d printed products and we've had trouble sourcing a vendor to print 100k+ units.
The Idea: A platform plastic and 3d printer companies.
Deep Dive: Similar to Alibaba, you would build a platform for companies to offer their services. Alibaba is seen as the "bottom of the barrel," you could implement a screening process and offer legitimate companies in select countries that offer 3d printing and plastic injection molding.
Validation: People will pay for better service, compared to Alibaba. You could source a few local companies and throw up a very niche listing site to see how it performs before investing a lot of time.
Monetization: You would charge companies a small monthly fee to signup to receive business on your platform.