Things to do

Miller’s record, field, and pair separators can only be single characters (e.g. newline, comma, equals sign), optionally allowing repeats on input (e.g. multiple spaces treated as one). It would be nice if strings were supported, e.g. "\n\n" paragraph-oriented record separation, or mix of space and tab for field separation.
Quoting isn’t supported in CSV. For example, Miller splits a,"b,c",d as a then "b then c" then d.
String-oriented functions such as sub, and Miller’s filter, could be made far more powerful if a regular-expression library were included.
Various to-do items are listed here.