Degica just announced that CAVE is coming to steam. The first game to be released is yet unknown, but Blackbird on the Shmups forum wrote:
“Well. I didn’t think we were going to announce this quite so quickly, but here we are. Shin is one of my co-workers, and this announcement is for real. CAVE is developing a game for PC. Without giving away too much, it’s a real arcade game and a fan favorite, so you can put your fears of bad mobile ports or freemium games aside. You can expect the same level of high quality seen in our other shooters like Crimzon Clover and XIIZeal.”
My “wild guess” would be Mushihimesama or Mushihimesama Futari 1.5.