In the US, Atari tried to sue someone who made an Asteroids clone back in 1981 and lost because Meteors, the clone had made some improvements on the idea of Asteroids (color, among other things). This cemented US legal precedent that you can't sue people for "ripping off" games so long as they make some meaningful change to it and aren't just making a direct knock-off.

This current case is in Japan, however, where the legal landscape is very different and companies need to be legally aggressive to maintain any rights to their IP from what I understand. I have no idea how that's going to go down.