Instead of just a wordpress plugin. I would vote for a general purpose method to integrate Flarum into other systems. For sure, Wordpress would be awesome.
But, I think Flarum would be awesome mixed with say, OctoberCRM too. I'm sure there's a lot of other frameworks or CRMs that people would like having Flarum integrated with. So like pwFoo said, a universal integration plugin of sorts would be awesome.