Given that we already have three or four overlapping, and two directly
competing, XEPs for the functionality I'm really uncomfortable with a new
one.
XEP-0135 still seems ideal and ideally designed to me. The objection seems
to be "it does not support notifications". Could you describe the use case
you see for notifications? Also, if needed, could this XEP not define only
the missing piece (notifications) rather than yet another new syntax for the
same data?