Kanji OpenType fonts have already grown, Chris. Take a look at Hiragino Mincho (bundled with Mac OS) or Kozuka Mincho Pro (Adobe, each font is over 4 MB with 15,444 glyphs).
I disagree with the comment that
“Opentype fonts can have many more glyphs and features than TT or T1 fonts”.
I can stick thousands of glyphs in a T1 font, give each
of them a different name, and extract them quite
nice