Currently, if a nation has a user named 'kitten', then the nation can't have any pages slugged/stubbed 'kitten' -- can we remove that restriction?
The simple way to deal with this is to just remove the username "kitten" from that person. You can do this from the "Public profile" tab on the person's record in the control panel.
It was a fair amount of work to allow people to have /username, which I think is a great feature, much more desirable than /user/username. And since it's so easy to reclaim a slug, I'm inclined to leave this feature as is.