Mybb and plugins for the most part is good but not great, I would like to see plugins for mybb that does similar fuctions as Geo IP, scrolling buttons, Adk Display Moderators to name a few, most of the addins I have seen for mybb have limited value and some should just be rolled together with others of reated functions.
Just post a request thread in the plugin requests forum with more details and see if somebody fancies taking them on then
Instead of telling people to create more plugins and themes, why don't you contribute submissions yourself? It's up to the community to create and maintain submissions. It would only make sense to do so as you're a part of the community.
* Wildcard wanted to say what Nathan said but probably wouldn't have done as good. 
I am a rookie at PHP and MyBB plugins, but even I can tell you that this ain't a picnic. There are a lot of things to consider because you have to be a programmer, a designer and an inventor all at once.
Even if you can't code, you can contribute by requesting specific plugins and giving authors more to go on that the name of it :s . . . why not take some time and really think about what you'd like the plugin to do and write a thread with your idea.
You might be surprised that a) it isn't so easy to come up with ideas for a plugin; b) when you do, making it work is a huge challenge and c) you find a little more respect for those that create free or paid plugins for this software.
