Going from Sakamoto Ryuichi, you should reach the natural extensions of that tribe that originated from Yellow Magic Orchestra and the other members (Harumi Hosono and Takahashi Yukihiro), Sakamoto's ex-wife Akiko Yano, and so on, including related bands like Sandii and the Sunsetz, P-Model...etc.
Psy S was a really interesting art-pop band with diverse influences from rock, electronic, classical, pop, lounge...etc. The male half went on to compose music for video games and created the famous Parappa the Rapper.
Barbee Boys was a friend to Psy S and also a very interesting band with a unique take on rock, with a female and male vocalist on sax.
Buck-Tick is an interesting visual rock band that started that whole movement. I guess you can call them goth in some ways.
Olivia Lufkin sort of rides the line between mainstream and indie, as some of her songs are definitely not mainstream, but some are used as anime theme songs.
Kanno Yoko is a composer for film/anime and her solo stuff is pretty interesting.