This is something that would be invaluable to users. A sample config with all the bells and whistles goes a long way toward demystifying the format, and gives a leg-up to those who need to set up their own, or do a bit of hand-editing.
I’d prefer HOCON as a format, but if you want to show off you could offer support for several.
Promotion / demotion permissions and associated rank ladder(s) would seem highly desirable. Storage of custom flags used by other plugins (eg number of homes) would also be nice. Prevention of cyclic inheritances and similar boondoggles is important too.
The thing I’d really like is a simple syntax for commands, along with some useful feedback to users when it isn’t correct. The hardest thing about permission systems is usually trying to grok the syntax, and live feedback is a great adjunct to good documentation