So, I recently built Kullervo who represents my homeland of Finland, and that got my thoughts drifting towards other things Kalevala. This train of thought collided with another about the upcoming grimoire weapon type.
To lay some foundation to the idea, in the Kalevala epic Väinämöinen was a demigod and the first man born at the creation of the world. He was a skilled musician, singer and wizard. In Kalevala singing and music was essentially how you cast spells. (During a singing battle against Joukahainen for instance, Väinämöinen sang him into a swamp.) There's a part in his story where he slays a massive pike and crafts a magical kantele from its jaw bone ('kantele' or 'kannel' is a finnish harp-like instrument). He played it so well that people, animals and even gods came to listen, and even the roughest of men were moved to tears. As a feat Väinämöinen also charmed the people of Pohjola to sleep to steal back a font of infinite wealth called Sampo. The kantele was lost to sea during a battle for the Sampo, but Väinämöinen later made a new one from birch, which is much less interesting.
In a nutshell the idea is this: a grimoire weapon based on the jaw bone kantele of Väinämöinen.
Though not a grimoire per se, I think it would fit how the grimoire weapon type has so far been shown to be held and perform attacks with. Though a kantele is a somewhat large instrument, there could be some creative liberties to make it fit a warframe's hand. The casting motion would remain more or less the same, with the warframe's fingers scraping the strings to play a note or two, which would convert into a magical void-attack.
A jaw bone harp would just be very interesting as a weapon and bring variety to a weapon type that's essentially going to be a collection of books.
It could have a charming or lulling ability as a reference to the myth. (Alt-fire perhaps?)
I haven't had time to think this much further, but I felt like sharing what I'd thought of so far.
