https://idolmaster-official.jp/news/01_16158
Just announced at their concert, THE IDOLM@STER SideM will be collabing with KING OF PRISM.

https://www.youtube.com/watch?v=7_03lKFGnwA

More details will be announced on a further date.

The characters collabing on the KinPri side are Over The Rainbow (Kouji, Hiro, Kazuki)
The characters collabing on the SideM side are S.E.M:
https://project-imas.wiki/S.E.M