I’m afraid I still don’t understand what you’re trying to do. You will have those other forums and sub forums, but they won’t be listed on the front page? What is the rationale? Maybe a plugin already exists to do what you want.
That screenshot is nothing like what you described earlier. What exactly are you trying to do? Do you want it like the screenshot with parent and child forums just laid out in different areas on the front page?
In order to accomplish what you want, with the positions of different forums laid out differently you are going to have to modify your front-page template file pretty heavily.
You’ll want to modify the ‘<?php if ( bb_forums() ) : ?>’ section so that it positions each forum in a different area of your page as it goes through the loop. It will likely get pretty tricky as you try to get the subforums underneath the parent forums since they may or may not be in that order in the loop.