2. If you have not a lot of categories/forums their is hardly anything to discuss about.
That's a misconception.
Not every category/forum needs to be ultra specific. Some places can be very general about things. Like say a forum that has a section for just music instead of one for every genre. The site may not need individual sections for rock, pop, classical, electronica, etc... because they either don't have as much discussion on it or it's not their main focus. This doesn't mean discussions about techno or dubstep can't happen... it just means there isn't a dedicated forum on the site for it and it alone. (Rather it just goes into the generic music section.)
Truthfully most sites would be better off not trying to have a hundred forums in them right from go simply because trying to populate all those sections with content takes time/effort and failing to do so makes the site look less active (or like it's dead/dying).
Also if there is a reasonable need for some forums then practical use of sub-forums should be used. Group up the sub-forums wherever possible. To stay with the music theme... Say the parent forum is "Music" then sub forums could be like:
- Rock (which could include classic rock, hard rock, etc...),
- Pop (possibly including J and K pop too),
- Metal (Death, heavy, glam, etc...),
- Country.
Anything not explicitly covered by one of the sub-forums could then just go in the music forum itself.
Though it all depends on the forum and user's focus. So if no one is really talking about Pop music at all (like less than 10 topics) then why bother have a forum for it? But if they were really talking about Classical then definitely have a section for it.
This way forums can keep their indexes clean/shorter and still offer places to talk about things.