Well if it was for fun, I’d invest in autonomous paving mothers (APMs). A self-driving solar-powered mobile furnace robot that eats sand and dirt and spits out interlocking solar-panel paving stones (whose power is fed back into the APM to help it along). As a necessity for holding back deserts, shoring against rising seawater, providing power and roads to desperate areas, and colonizing space (we need dust-free tarmacs to live on), it could change mankind.
I also like the idea of making 100,000 solar-powered self-driving surfboards to roam all the shores of earth so there’s always something to play with when you take your kids to the beach (I’m tired of fumbling with inflatable ducks).
And if it’s for profit I’d invest in advanced life-size humanoid executives to act as the research platform for a mass inexpensive humanoid robot market. Start them out as executive helpers (a walking I-phone that can bring you coffee), then scale to tele-robo-tourism (why not take a walking tour of Hawaii before visiting?), and shopping (your robot avatar working a market far away).
Data from this development could then be used to optimize the ultimate robot market, a $1000 soft-plasic humanoid nurse that will keep the house in order, fetch and cook food, dispense medicine, then put itself away in a closet. An affordable option to give people the dignity to live at home comfortably through their dotage without burden.
Yes, there might be a demand. A good investment I think.