Plugins will highly depend on the application you're writing them for. That is, one application's plugins may bear very little resemblance to another's. The only real similarity would perhaps be that they both [are designed to] extend functionality in some way, but how it's done in each case can be significantly different.
As for PHP tutorials, there's many of them across the internet , and I believe there are already some MyBB plugin guides around the place.
So I'm not sure about this, but if there's any difference in me putting in all the effort to do it, then please say so.
Thanks for the suggestion regardless