What are you interested in? Administrating a forum can become a burden if you're not interested in the topic that the forum is based around (i.e, Say you love dogs and know nothing about astronomy. You wouldn't want to have to administrate a forum talking about the particle physics involved in multidimensional space now would you?) I'd say make a forum with a topic that you are interested in. If it was me, I would make the topic be over programming languages and securities.
Now if you are allowing people to pay you to allow them to use your server for their forums, I'd say let them make the forums around their own topic (while still keeping it legal, i.e. no warez and crap like that)