I suggest that a new directory, $HOME/.var, is defined in the XDG Base
directory spec. The following is inspired by the wording in FHS
http://www.pathname.com/fhs/pub/fhs-2.3.html#THEVARHIERARCHY

"$HOME/.var contains variable data files. This includes transient and
other temporary files, for example trash and cache files."

If this is included, I suggest the default of $XDG_CACHE_HOME is changed
from $HOME/.cache to $HOME/.var/cache, to mirror usage of /var/cache

A simple but important step to adoption could be if major distributions
included .var in /etc/skel, so that it is created by default in new home
directories. If the trash spec uses this as well (and actually gets
implemented), it think it stands a good chance to get more broadly adopted

Claes