I'm proud to say, you can now easily create sub categories in your forums!
And there is more to that, you can now manage all your categories from the live site!
Go to your published site
Click on the new 'Manage Categories' icon
Now you can manage your categories (reorder, edit, delete, create new)
And of course, you can now make any category a subcategory and create new categories easily.
You can also do it from the Forum app settings in the Editor.
Let us know what you think - and share with us your forums :)