I'm sure it's come up before, but being able to input new rank themes would be lovely.
I think the tricky part is the implementation. One rank per line doesn't work since we need four entries per line (naval rank, abbr., ground rank, abbr.), and using space as a separator won't work since ranks often have more than one word in their titles. I would suggest using a CSV format since ranks pretty rarely have commas, I don't think any of the current defaults use commas for example.