I have a very bare bones idea and it's basically an homage to one of my favorite video game characters, that being Frog from Chrono Trigger, who for those of you who don't know, is a knight that was turned into an anthropomorphic frog by an evil mage.
The idea would kind of be the reverse of that. It would be a very fairy tale esque story of a frog that lived in a pond that this princess would often go to to relax. She would feed the geese and pick flowers or whatever the hell girls do, but she would also sometimes read. Or maybe if she were younger, she would be read to. Anyway, the book is about a knight who goes on a noble quest to save a princess and it has these neat hand painted illustrations and the frog would sit and listen to the story and look at the pictures. Then, one day when the young princess, I think I like her being like a little girl of maybe ten, comes to the pond alone, she gets kidnapped by goblins working for the evil wizard or something. I dunno. Anyway, the frog is the only one who sees it, so he sets off to rescue her. But, you know, he's a frog.
I figure the actual gameplay would be sidescroller style and starting out, the frog can jump on some enemies, but most are too dangerous or two big. But maybe with the intervention of a fairy godmother or something, or maybe there was magic in the book th little girl had and she's tearing up pieces of it to leave a trail, and maybe when you find them as the frog, the magic of it turns you into a knight. Still a frog, but like a frog knight. And then the gameplay would change quite a bit. I'm thinking as the frog the game is a little more Mario, but as the knight, the game gets a little more Castlevania or something. You suddenly have a lot more options and are actually a threat to the enemy.
I don't really know how I would balance the parts to make it not annoying, but I think it might be kind of fun.