i don't think the sub-forum idea is actually a good solution. Even to the point where you have to choose which is the default view.
It could be possible to implement a tagging system, and allow filtering via tags. still a single unified active forum, but very simple to click the top few favourite tags, like 'diplomacy', 'game adverts', 'best trolling', 'politics', 'religion', 'school of war', etc
Something user created and defined. Perhaps with a reddit like system where +1s actually mean something ('hottest topic of the moment', where a +1 effectively means an hour is added to the threads effective post time for sorting by time, so the most +1'd of recent threads comes to the top...)
This was something which was discussed prior to implement +1s and it was decided that we could use look at how the +1 system works and see if it can be used to filter by popularity...
I would really like to see a 'school of war' tag which i could click on to get access to all the school of war threads... But also keeping the forum in one place, one default, one front page, is important. IMHO.
'I think it's possible there would be more Dip threads if they weren't buried by the other content.' - other, more popular, content. FYI I do keep a track of any diplomacy threads i find and link to them here: https://sites.google.com/site/webdiplomacylinks/
Feel free to create article and i'll post them, its not a very active site, as you can see.