Hi there. First of all Flarum is the best forum up to date. You guys did an exceptional job! Wish you all luck and success with the thing!
My question here is about layout that one could integrate into his own website. You see, one of disadvantages of many forums out there is that whenever anyone integrates them into their websites they look absolutely different. And the "difference" starts with the header.
I strongly believe that when one develops a "system"-website, there should be one header for all the functions (static pages, blog, forum etc). Therefore I would suggest you to consider an integration layout switch for any theme there can be - thus header to be hidden and all the header functions would stay on sidebar, with possibility to process a centralised login with other CMSs (I am using Kirby now and considering Flarum for forum).