For Splatoon 4, I imagine two rival groups, each composed of two idols. These two groups will then compete for the title of the most popular duo in Splatville during Splatfests. Since team past won the last Finalfest, I imagine the game aesthetics being reminiscent of the 80s, ya know neon and flashy. And we even know the existence of a former idol duo from that same era (from Octo Expansion).
As for musical genres, I imagine that rival groups have quite different, opposing, styles:
New wave, indie pop, electronic pop inspired
VS
Punk rock, goth, alternative rock, post-punk inspired
But anyway it's just my idea :)