How about a 'Newbie RP' area?  Brainstorming here, so this needs to be tweaked.  Essentially, a free for all, do whatever you want forum stickied/tacked onto the Introductions forum.  It's aim is to bring out the RPer in newbies and get that engine running.  Everyone here has very few posts - it's to make them comfortable and not be daunted by those with 1000+ posters.  Easy-to-join threads for Free/Light, Casual/Medium-length and Advanced/Long-length.  Eventually, those who have gotten themselves comfortable can start to check our real RP forums.  Caveat: we might get a huge clump of newbies becoming oldbies staying in the new section.  It could be a very general thread idea like "You come to a door, what do you do?".  I have no idea how to handle the diversion of plots within however, so build upon this or take it apart. :)

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A tangent that might also help in the onboarding:

On an old forum I visited, I was very motivated by the leveling system they had.  10 points = Soldier, 50 points = General and so on.  I was no longer the newbie with 5 posts.  I was someone reaching the Soldier rank.  So I talked and participated more.  It helped develop my forum's persona instead of letting me waste away in silence.  The cherry on top was that our ranks were colour coded and had picture associated with it - like those strips you see on the Navy's shoulders.  The visual picture element meant that we could instantly see each other's ranks.  It was a fun rank - nothing competitive.

Other smaller ideas was a points system where you could use your post count to buy armor.  It was just text.  E.g. Under my name, I had the equipment of kitty ears for 20 points and evening gown for 50 points.  It was more boring, but some of us like the cosplay element.