My usual reason for not doing something is that I'd rather work with someone else, so the available enthusiasm selects my projects. In this field, I would include a trailer-mounted rig to convent logging slash and other biomass that emits CO2 on organic breakdown into biochar. I'm thinking of an oven that can hold two SS containers at a time for continuous production without burning any carbon.
I'm also interested in building a wood stove as a low-pressure, solid fuel gas turbine. Using only cheap materials, I'm sure it could run its own fan to work with a condenser and cool outlet instead of a chimney, but I'm hopeful that it could also put out 10% as electricity. It does the work of a gasifier and engine, but it takes exotic materials to equal their efficiency. A car turbocharger might work to spin a magnet.
The big project that I'd fund if I won the lottery is a permaculture robot. Something produced by 3-D printing might be best, because we need enough to replace all the no-till farm equipment. I'd aim for something shoebox size, with the ability to keep track of every plant within a plot, and plan for maximum production using AI from the 'net. It would recognize, save and offer for trade the best seed, and harvest each plant at its peak while usually mulching the waste on the spot. Weeding would probably be done with a nick from a laser. The project could probably finance itself just from starting with a weeder that will zap anything not growing through a special ring. Tomato picking might take some development. The robot would probably have a little tool shed with a solar panel on a pole to recharge its batteries. It could probably navigate using signals from the markers at the corners of the plot.