Perhaps this has already been considered, but I have an
idea that would make neat addition to python:

Appending a string to another string can be done with
+= or .join(), but currently, there is no elegant way
to prepend to a string.

How about something like-
string.prepend(list.pop(0)

?

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-0.
What's wrong with
>>> string = something + string

Also, the above conveys the notion that this is an expensive
operation (when done multiple times, e.g. in a loop) better
than a str method.

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All string concatenation in Python creates a new string and
leaves the original untouched.  So, there are no append
operations.  Likewise, the can be no prepend operations. 
Instead, there are only a+b operations whose result can be
assigned to 'a'  to give the effect of an append, to 'b' to
give the effect of a prepend, and to 'c' to create a new
variable leaving the inputs untouched.