Yesss the Mikus are great, what a fun promotion. I thought about posting about it in the news thread
The art is done by familiar artists too. Take helped with character design Gen 7-9 and Mizutani is pretty prolific in the series being a TCG illustrator, Pokemon & character designer, and drew the official artwork for an assortment of Gen 6-on trainers.
Assuming they keep the music motif I'm guessing...
Poison: Toxtricity (literally a rock star pokemon, bet the outfit would be a mix of both)
Dragon: Altaria (song bird, owned by Lessia a performer)
Dark: Obstagoon (the singer of the galar band)
Flying: Farfetch'd (for the same reason its in the promo image: llevan polka)
Normal: Jigglypuff (the og singer)
Fighting: I'm going to go with Hisuian Lilligant or Kommo-o. Both are explicitly based on performance, so it just depends on if they want to go with ballet or haka.
Ground: Someone from the Tympole line; Tympole in particular has a slight music note motif and the whole line is based on vibrating and stuff, so I think that's a good thing to work with
Rock: So I was going to say Roggenrola, because even in Japan the pun is basically "rock and roll", but I'm going to pivot into a reference like Farfetch'd and say Rhyperior, because it shoots out rocks. Perhaps black ones. Like Black Rock Shooter
Bug: Kricketune (Conductor)
Ghost: Misdreavus (or Mismagius). I always kind of associated Perish Song with Misdreavus more than other Pokemon; if they went with Mismagius I think it'd be cute to have an Iono crossover
Ice: Lapras. It got associated with Sing so much it got a gmax entirely based around it. Mr Rime would be funnier though
Fairy: Diancie. Twin tailed princess with an extravagant mega form I dunno. Just kind of fits. World is mine.
Steel: There's not really a performer here, so they could go with something "girly" like Mawile, Magearna or Tinkaton. This might just be a "go with someone cool" type deal, so maybe Lucario or Gholdengo.
Electric: Pikachu sung once. we're not escaping PIkachu.