Kerning already supported ... text would look broken without it. glyph composition and rtl/ttb/btt - I don't support that now, but I want to support it eventually.
Hebrew and Arabic are the only major rtl languages I want to support. Almost no one uses ttb/btt in computer UIs, CJK use it in signage and books though. Even traditional Mongolian which is hard ttb is replaced by Cyrillic letters on computers. Doing boustrophedon direction could be fun... but probably just as pointless as Egyptian hieroglyphs pointing their heads in correct orientations.
