There's a line between wants and possibilities. Like you said, you don't want people to be able to 'hijack' pages such as /blog, /signin, etc. but vanity URLs are nice for us users when we want to share our works. So figured my suggestion drew the line.
Easy to remember/type but non-related page URLs couldn't get hijacked
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Edits:
After thinking about it, you wouldn't even need AJAX to handle it. just a bit of JS that redirects from #username to /profile/username, though you'd have to alter how profiles are currently accessed, that is currently user profiles are retrieved using their userid number.
It's not a technical problem, but a UX/value question. I think a /username URL is much more valuable to users and is pretty much the norm online. I mainly just need to come up with a good list of stop words. I may also need to request users who have names that conflict with the stop word list to change their name. And finally I'll need to make this something you can change via account settings.
Yes, I've been considering that, but what concerns me is users taking practical urls away. For example somebody might take /blog or any number of URLs I can't think of now but may want to use in the future.