Is Brandon Sanderson planning to write a book following Hoid's story? I heard somewhere that he was, but I'm not sure.
Dragonsteel won't be about Hoid exclusively, but it will be set on his homeworld and it will feature him as a major character/narrator.
This story (the story of the shattering of Adonalsium, as told by Hoid) is next-to-last in my sequence of cosmere novels (though it’s first chronologically). So don’t expect it until Stormlight 10 is done.
Status: A long way off. Though it might still beat that one book by that other author.
State of the Sanderson - 2016
And then he is supposed to be a main character towards the end of Mistborn.
When are we going to get Hoid’s book?
Hoid has 6 books, they are the 3 books of Dragonsteel, which are prequels and the last Mistborn trilogy of the nine book arc will have him as a main character. I won't say they're "his" books, but he is one of the primary protagonists.
Words of Radiance Signing - Arcanum
Yes, Brandon is planning on writing a book series called Dragonsteel which is Hoid's origin story as well as the story of
the Shattering of Adonalsium
Brandon's 2016 State of the Sanderson says this will be "as told by Hoid" which I take to mean he's either the POV character or narrator. The 2017 State of the Sanderson simply says it's Hoid's origin story.
Either way, he's been pretty consistent that it won't be written until The Stormlight Archive is done, so we're probably looking at a few decades before we see it.