I'm wanting to run a category where the players work as a team to speedrun all the boards together. Does something like this already exist? If not, would that be a sub-category I can propose?

You could also ask in the category extensions.

https://www.speedrun.com/mariopartyce/forum

Bird and I were thinking of adding this as one of the sub categories (not one of the main 5), but we haven't really talked about it in forever so Bird and I will discuss it again at some point since there seems to be some common interest

Awesome Thanks! Looking forward to your verdict