Whimsy Vessel Theory
What if the vessel for Whimsy is actually a young child? Around 5-8 old range.
We know next to nothing about the Whimsy Shard besides a few offhand remarks. It seems that Whimsy is a bit of a chaotic force and not particularly consistent. For all the MtG players here we also have this WoB that gives a little bit about what color Whismy would be.
KandraAllomancer
What Magic The Gathering colors would the Shards introduced in Rhythm of War or after be?
Brandon Sanderson
I have to RAFO that for now, because it kind of depends.
But Whimsy is pure red.
- Dragonsteel 2022 (Dec. 13, 2022)
I thought that Brandon saying the Shard was 'pure red' was interesting. I read an interesting description from the mtg fandom wiki that I thought could describe the Shard and intentions driving the vessel well.
Red thinks that all you have to do is listen to your heart and simply act accordingly, letting your emotions guide you. Red loves life much more than any other color and so it believes that all people must live it to the fullest. Red believes that life is an adventure, and that it would be much more fun if everyone stopped caring about rules, laws, and personal appearances and just spent their time indulging their desires through experience. Red doesn't live its life questioning choices it has made and lives in the moment; Red is spontaneous and embraces every adventure put before it. Red is often charismatic, even as its antics upturn the established order.
When I read this description of the the MtG color red and tried to apply it to what the Vessel could be like I thought it would be really interesting if this shard had a child Vessel. A young child would be much more likely to maintain there whimsy than someone as they grew older in the dangerous cosmere The short-term need for immediate gratification, not understanding why rules and laws are important, and lack of perspective to understand the need for a longer term plan could be because of both the Shard, but also because of how young the Vessel was when they ascended. I speculate that this would trap the Vessel at the stage they ascended, stunting their growth. I think Brandon would enjoy writing about a child who suddenly could go from playing in a sandbox to playing in a sandbox the size of the universe. While the adults below had to deal with the spontaneous and childlike God who couldn't understand the damage they are doing.
My last point would be that Harmony didn't find them particularly helpful when he reached out to discuss the Odium Shard with them. The Shard intent alone would make it hard to communicate effectively with I would imagine. However in addition to that I imagine Sazed also realized a young child was in control, and he knew they wouldn't be useful in any cosmere war with Odium or as an option for council.
One question that would be interesting to discover is how a child came to be in possession of Whimsy. Maybe they ascended on Yolen, or maybe made it's way to this vessel after something happened with the original ascending vessel. I don't have a great theory but I bet Brandon could find a way to make a child ascending possible and interesting, plus would enjoy writing about the consequences of a whimsical child in control of a Shard.
TL:DR Whimsy Vessel ascended at a very young age and the Whimsy shard is being controlled by a child.