Why create any trail? 🙂 You could join the dots when caching anywhere, and yes, sometimes that’s fun.
Challacombe has a nice circular walk, which is sometimes used for events, and some excellent history with Birch tor and Golden Dagger mines. Could even have a bonus cache sited near to Plymouth museum where the dagger was last seen before it was bombed.
But as the letterbox charity walks show, circular walks can be done anywhere on the open moor.