I'll start.

A programming language with the ability to dispatch on version could be very interesting, and possibly allow for safer upgrades on a per function basis.

Still it feels like this should be part of an interface between a PL and some other set of packaging options. A convention for where to place libraries (with versions) like /lib is basically the solution. But I guess the various communities don't really want to get into a turf war, so maybe it's /lib/ruby and /lib/python in addition to the rest?