Rivals would be interesting, though performing together would be cool to see happen too (presumably in a non-idol-wars context since with Sheryl gone Maria's pretty much the backbone of Sparkle Motion now). Also, that sounds like pretty standard shonen rival "surprisingly decent" behaviour...

WELL I mean if any show was going to pull off card games while singing, Vanguard should've done it with how good some of their character songs are tbh. I would totally watch idol card games: the series, though.