I would love a way to store persistent data that doesn't get replicated. This likely wouldn't make as much sense on PlayerObjects, but it would be great to have either a flag or a section in PlayerData that's scoped only to the local user, for storing more private data.
This would be great for things like a "last entered code" for admin panels, for example, where it improves UX but you wouldn't want it replicated out to all clients in an instance.
I can forsee one potential issue with this however, users storing potentially sensitive information in these keys (such as passwords) so this would likely need to come with some kind of disclaimer.