I managed to beat the game on Friday but I didn't find anything noteworthy on actual transformations but I did find a short comic about one of Snake's compatriots inside Isabelle's body.
https://kira-vera.tumblr.com/post/17...y-pupper#notes And another joke about Ridley being Samus' Now with Bowser in Mario's:
https://kira-vera.tumblr.com/post/17...emselves#notes
Other than here's the last female spirits in non-human bodies that I managed to find out after I finished the Adventure mode (note: some of the spirits after Madama Butterfly may be exclusive to the Spirit Board).
Midna from Twilight Princess in Bayonetta's body
Mipha the Zora from Breath of the Wild is in an Inkling's body
Rouge the Bat in Fox's body
Mother Brain from Metroid is in Samus' (I just figured it could be the dinosaur like form in her body instead of her brain form)
Madama Butterfly the summon from Bayonetta's Smash attacks is in Bayonetta's body
Serena that Chihuahua dog goddess from Animal Crossing is in Palutena's body
Cereza the little girl from Bayonetta 1 is in Isabelle's body
-From this point on I ended up thinking about combining bodies into one for Spirits that are partnered with non-human characters to help them out, which could also be good for TFs.
Minerva from Fire Emblem has Lucina and Charizard for her battle.
Camilla from Fire Emblem has Corrin (who could be female) and also has Charizard for battle
Grandrayde has Samus and Pikachu in the battle.
Wart from Mario Bros. 2 has King K. Rool and Peach for battle.
Power Star from Mario, the one that gives you invincibility, is in Rosalina's body.
Perry the Parosol from Peach's game is in Peach's body
Nia from Xenoblade Chronicles 2 has Mii and Incineroar for her battle.
Loonsley from Line-Up in Isabelle's body
The Reporter and Incineroar are both in Yumi from Fatal Frame and Pika Libre, which you could combine them with all four of them in different ways.
ST Falcon is in Samus
Arcreus is in Corrin's body (male but it could be the female one too if you choose to)
Cuccoo which is a female chicken from Link are in Falco's body
Meowth is in Isabelle's body
Now the rest of the Pokémon combinations for now, which again as species of each Pokémon can also be female.
Gardevoir is in Rosalina's body
Lunala is in Ridley's body
Solgaleo is in Incineroar's body
Starly from Starfy is in Jigglypuff
Eevee is in Yoshi's body
Gengar is in Donkey Kong's body
Porygon is in Wireframe Little Mac's body
Chansey is in Jigglypuff's body
Ampharous is in Pikachu's body who's giant
Miltank is in Jigglypuff's body
Moltres Articuno and Zapdos are each inside a Lucas body
Kanagaskahn is in Bowser's body fighting with Iggy the Koopaling.
Wheezing is in Wario's body
Venasaur is in Ivysaur's body and Blastoise is in Squirtle's body, kind of redundant but I think it's a good combination idea.
Wobbuffet both female and male are in King K. Rool's body for those that don't know a female Wobbuffet has lipstick.
Smeargle is in an Inkling's body (both characters in this instance can be female though I think he was in a male Inkling's body in the game itself)
Ho-Oh is in Charizard's body
Shedninja is in Greninja's body
Groudon is in Bowser's body
Giratina is in Game and Watch's body
Dedenne is in Pichu's body
Zygarde 50% is in Yoshi's body
Rowlet is in Ivysaur's body
And that's pretty much it for all the spirits I can dig up. I'm pretty sure there's a couple of others I missed because some like Serena the Chihuahua Goddess is exclusive to the Spirit Board. Well good luck everyone who wants to do all of this including the ones that like to do certain ones.