I would like an interface where the hrc is loaded and each bone is labeled (i.e. hip root) and has a box beside it where the bone length is displayed. This box can then be easily edited using the numpad on the keyboard to input whatever length is desired. It would also be helpful for them to be in a list format so that multiple bone lengths can be displayed at once so scrolling is minimized. For my purposes, it would be IMMENSELY helpful to be able to temporarily store lengths so that with the click of a button, the saved lengths can be applied to the new hrc without having to manually type each one in. So if I want to change hip root from 1.0 to 1.2 and apply it to others, I could click a box beside the length to save whatever number has been input, once a new hrc is loaded, another box can be pushed to load 1.2 in place of whatever number the hrc originally had. This is most beneficial for skeletons like Cloud where there are 64 other models that have his exact same structure.
As far as adding new bones that don't do anything, that may throw errors in kimera, but should work fine in game. If it's easy to do, it could be worth trying, at least until a proper converter is made for them