Perhaps a 100% 'redesign' of a platform already available would meet that kind of description. Suppose we make a schematic for an aluminum breech for a 13xx. This could be coupled with a 13xx/22xx PCP conversion, one we design and release as an open schematic. Finally add to that a 13xx 2 stage trigger schematic and you have a functional pistol you've made in your shop. I think this woud be capable with only a lathe + milling capability, or a separate mill. The hurdle would probably be tooling up on a budget.
With this setup there is no risk of the base design getting stolen since we're just making custom parts. The parts of this pistol are both simple and small lending themselves to home shop machining. On top of that, it seems we have forum members very familiar with this family of airguns =) I think this might be a way to get parts on paper quickly and generate interest in the forum and future projects.
Onto a different topic. I came here hunting for an open source design for a FT or 10m match PCP rifle design to use as a test bed for some ultra low muzzle energy long range testing. (~2J muzzle energy) I thought HM Buckley's MPA design would be the perfect thing for me until I saw the price tag. Nearly $80 to get it, and thats if you can even find somone who has one. As a student that was out of my reach. I'd also like to see how more advanced structures work like active stabilizers, methods for reducing movement inside the action before the shot, and quality in tank regulators. I realize a lot of the challenge of this would be not how they work, but how to build them in the home shop. (electronic trigger? now im dreaming)
Just some ideas I've had bouncing around since its late
EDIT: After I woke up I remembered something I forgot to mention. I'm not sure designing the pistol and then trying to add features is thte right way to go about it. I think some of the features we want to add might end up dictating the size/shape of a particular part and this could be problematic. Perhaps a different way to go about doing this would be to identify features from other highly respected platforms and decide how this could be implemented. Someone already mentioned this but I felt like it needed to be emphasized.