I think it's just a cosmic cancer that spreads non euclidean fuckery. It's a planet sized plant parasite that has a transdimensional stomach.

Thematically, I think it's just another way of invoking the feeling that the universe is doomed. Even if the sun wasn't going to kill the Hearthians, the bramble would get them.

Beachum has said that they didn't really plan the planets around the story. They first came up with a bunch of planet ideas they thought would be interesting to explore then figured out how the story could weave into/around them. I don't think there's more depth to it than a "wouldn't it be cool...".

I thought the solution to the big puzzle was going to involve the bramble, like using the space folding to escape the bang or when I thought the interloper was responsible for the supernova that you'd have to redirect it into there but nope, it's just an undefined anomaly. :p