Guy Steele's keynote at the 1998 ACM OOPSLA conference on "Growing a Language" discusses the importance of and issues associated with designing a programming language that can be grown by its users.
Link to PDF
http://www.cs.virginia.edu/~evans/cs655/readings/steele.pdf
Link to PDF http://www.cs.virginia.edu/~evans/cs655/readings/steele.pdf