First, as you can see from my post count I am extremely new here so take my comments in light of the fact that I'm a serious newbie.
I hate to point to another forum, especially one that I absolutely DESPISE, but at least in this regard the warrior forum does a pretty good job of turning every member of the forum into a sort of "admin". Posts that get flagged more than a few times are dropped automatically. Perhaps you could take some of their ideas? (without turning into a cess pool of negativity like they've become)
Revitalising the forums a bit to move with the times: opening some new forums (perhaps Git, SASS/LESS, node.js)
In my opinion the problem isn't too few forums, it's too many. You're fracturing the user base across too broad of a space. This means that no one post is likely to see very many views simply because your active user base is all over the place. Simplifying the forum into only three "web design", "web programming", and "web marketing" would consolidate the visitors and would likely cause much more discussion because more people would be compressed into less space.
Kill some of the dinosaur forums which are spam traps (SEO comes immediately to mind)
Again, see my comments above for turning everyone into a form of forum admin and the "killing of the forums" idea
Overhaul the current incentive system - develop on the pips idea so that there are extra incentives for making high quality posts
This is probably a good idea. IF you also get "points" somehow simply for voting on people's posts. Sort of like a "Karma" thing. You get karma for making good posts, lose it for making poor posts, but you can also gain karma by voting on posts.
Start some short fun quizzes and graphics wars - competitions that could be done in a lunch hour, rather than the current ones which sometimes run for a week or more.
Love this idea.
Have a repository for valuable go-to threads
For this, all you'd need is a way of pulling the posts with the highest amount of quality (most high votes or some such). That way this repository is something that is actually built by the entire community.
Have a fun 'who are we?' section. Forum Staff/SPHQ staff/SP authors spotlight: Interviews, what they do, how they work, how they plan, how they interact.
Eh ... this one sort of exists already through the profiles. If you wanted to do something like this, it might be kind of cool to put together a sort of monthly "tv show" (youtube?) that does some of this and embed it into the forum.
Have a greater presence from SPHQ staff. Perhaps expert Q&A sessions. Interviews on subjects that you request.
Do this as video ... or possibly a webinar (anymeeting.com/ustream.tv) and it would probably be pretty cool. Again, just embed into the forum.
A new forum called "News From Our Members" where you get to talk about what you're working on and blow your own horn a bit. It would be moderated so that it doesn't become a spam trap, but it would allow you to showcase new and exciting things.
Could be interesting. This might be something to think about ... but again, I think your overall issue is too many forums. There's too many different places to go and visit and too few people (even still) are using your RSS feed to simplify things. Like I said my biggest suggestion is to VASTLY reduce the number of forums.