A two person boat plan and/or kit would be phenomenal. Something similar to the Explorer 42 perhaps.. I think the boat plans are probably the most helpful for folks, once one learns how to assemble them, they can come up with their own accessories. Getting the patterns for a new boat design would be a lot harder.
Inflatable PFD would be pretty slick as well, we all know that is something that is still missing from the market. An ultralight version could be an inflatable horseshoe with a couple straps, a more comfortable one might be a nylon vest with several inflatable inserts..