First regarding an author base:
There are currently plugins devoted to this, and just this.
Actually more than one.
This is such a small little feature it just seems right to tuck it right underneath the category and tags base on the permalinks page.
Secondly regarding extensions:
I keep reading more and more that search engines are taking into account filename extensions into their caching policies.
That being said it makes sense to allow file extensions in the url rewrite of pages, posts, and even attachments. Right now we can do this for posts but page slugs are sanitized a little far beyond to allow this. I'm pretty sure dots turn to dashes. I have not yet found a good way to edit attachment slugs.