If I were planning a high quality kit I'd be considering what the market is and what's already available. Of course being able to use an existing high quality lock is almost a requirement , as well as an existing profile barrel. I'd rule out anything that really requires engraving to look good or right, because some of the market for an easy to assemble kit will be inexperienced builders.
Smooth rifles seem hot now and are fun and versatile. I'm not sure what the Hawken market is. I believe a relatively inexpensive trade gun in 24 or 28 ga with a really light barrel would sell really well, but it might not need this quality of kit offering. English walnut is out, perhaps, so that may limit options. As far as military guns are concerned, the Bess is the most desirable but I am not sure who wants one if they do not re-enact Rev War events.
In considering such things I have to try to get away from what I like and try to gauge what will be eagerly received.