i think that djangobb should have subforums.
so, by that we should develop our own model or use one of that:
http://django-mptt.github.com/django-mptt/overview.html#what-is-django-mptt
https://tabo.pe/projects/django-treebeard/docs/1.61/

i think that mptt will be best option, cos' we have full featured administration of tree

django-treebeard more interesting for me ) but it was when mptt stoped development

i don't saw any profit of using this libs for this task. Forum -> subforum - this only 2 levels tree and can be done via simple FK

but with this libs we can make:
Forum -> subforum -> subsubforum -> ... -> sub...subforum

one level subforums are not that interested as multisubforums

I doubt about the usability of this)
show me forum with this feauture that used it in practice

phpbb -> http://www.phpbb.com/community/viewforum.php?f=69
ipb -> http://forum.invisionize.pl/forum/87-tutoriale-i-porady/
vbulletin -> http://www.vbulletin.com/forum/forumdisplay.php/40-Pre-2.0.1-bugs

so it's commonly used
when we will have multisubforum system, we can also add forum type (link, category, forum)

ok, maybe at 0.2 version)

for subforums we can use mptt with feincms editor