From the 10.000 ft point of view, I see the following
possibilities:
o Decide that support of privacy & international usernames
is not necessary in IKEv2.
o Remove the privacy and international username (not domain)
parts from RFC 2486bis.
o Change the international username part so that instead of
UTF-8, it uses a IDN-like ASCII mapping which can represent
non-ASCII characters but it looks like ASCII to the carrier.
Either remove the privacy feature or just say it can not
be used in all carrier protocols.
o Define the new IKEv2 ID type as Yoshihiro suggests
below.