It seems that almost no one has figured out how to develop Sunfire into a character who isn't just 40% national pride, 30% arrogance and 30% anger or they don't want to. That's probably why he isn't used more often in the X-Men titles.
It might be interesting to see Sunfire and his sizable national pride maybe have to face some of the real social problems facing modern Japan along with his superheroing. Stuff like the aging population, inadequate childcare and foster care systems, mental health stigma, the hikkikomori phenomenon etc. in a miniseries or something.