minor mode for highlighting character classes such as non-ascii (bug 47455)

[Roland Winkler]

In the context of bug 47455 the idea emerged that it can be useful
to have a minor mode that highlights non-ascii characters.  I want
to ask hear for feedback: does such a mode already exist?  Can it be
useful to have this minor mode more general such that the highlighted
character classes become configurable?  Are there possible use cases
for this beyond non-ascii characters?