Before I explain my theory if you can even call it that, I'd like to forewarn you that there the connections I've seen and will share are very loose and are very much correlational rather than causational.
Hoid is the narrator of Tress and the Emerald Sea and Yumi and the Nightmare Painter. So for both of these stories he is recounting them to potentially different audiences at different points of time. Many of the "stories" we've heard from Hoid in the past were legends, folklore of some kind, so that is to say that while they could have actually happened, most likely they either didn't happen and are just stories he recounts for others to hear and learn from, OR they did happen but the details have been changed or dramatized over time. Regardless, as far as we can tell the events in Tress and Yumi definitely happened, and Hoid is most likely telling them as they happened exactly at least from what he experienced.
That makes me think that there is perhaps some common thread between the stories Hoid shares and why Brandon is choosing to have Hoid share multiple stories in this form to Us, and to Hoid's audience. Yesterday it was announced that another book like this is coming out next year, "The Fires of December". While don't know much about it yet, Brandon did just do a reading from it, and something stood out to me from it which is this. After December's husband Bark discovered a vein of silver which made him stupid rich as the Reeve (who is probably the King) paid him a handsome sum of money for discovering it. Where December lives there is a river of blood it seems which is the blood of a demon? Not sure what that is or what it means, but in this reading in a sort of aside it said that around this same time the river "was growing darker, losing its violet beauty, taking on a sickly blackness". Now I'm sure this detail will make more sense when the book comes out, but lately I've been really interested with the color black in the cosmere.
It seems that Brandon is very intentional with his choses of color, especially when it comes to investiture or things directly tied to investiture. Now would it be farfetched to assume that a river that is made up of the blood of demon may be somewhat highly invested? Now the color black is often associated with Ruin, but Brandon himself has stated that Ruin's color mainly had to do with opposition to Preservation and that in other circumstances his color may have been different. Black is also closely associated with midnight essence, and seems to be connected with the consumption/corruption of investiture (although red is also more closely tied to corruption). The best examples of this consumption is Nightblood and the Father Machine which both consume investiture and what remains is a type of black smoke for nightblood, or the shroud for the Father machine. Midnight essence similarly deals with the consumption of investiture as it is controlled through the luhel bond. Now with The Fires of December, there is already in an early passage in the novel a substance with similar features to midnight essence, sickly black, growing darker day by day?
Basically, my theory is that the common thread between these stories from Hoid, and perhaps the reason for him visiting these planets is tied directly to midnight essence and other forms of black investiture consumption. Or perhaps that isn't the reason for Hoid's visits, maybe he doesn't have his eye on this at all, but maybe Brandon wants us to be paying attention to this connection.
I recognize there are very loose threads connecting this all together, but I just wanted to share my thoughts and see what you all think. I'm no expert on these topics, so if any of you have other examples, relevant WoB's or thoughts to share whether to expand on this idea, or to put it down, I'm happy to hear them all!