I'd like to hear Kana Otsubo (Spangle call Lilli line) and Yumi Uchimura (la la larks, ex-school food punishment) make a 'proper' collab together. I say 'proper' because Yumi did previously provide vocals for a SCLL pseudo-cover not too long ago, but that track didn't feature any of Otsubo's vocals.
I reckon Robin Aoki (downy) could work really well as a guest-vocalist for THE NOVEMBERS. I know he already worked with them as a producer on zeitgeist (and co-wrote the title track too, I believe, which is probably why Kobayashi's vocal delivery is so downy-esque in that song), but I'd imagine his and Kobayashi's voices could mix together really well while still fitting THE NOVEMBERS' sound quite easily.
Other than that, there are quite a few people that I'd like to hear collaborate more despite not having a specific person in mind. Kyo (DIR EN GREY, sukekiyo), TK ( 凛として時雨), miya (375, ex-BLEACH), and Masayuki Hasuo (385, ex-school food punishment) could all benefit a lot from taking part in some experimental collabs in my opinion. I also think Miyavi's best work as of late has come whenever he's collaborated with others.