Personally, I'm probably the most curious about forts - it was eluded to, but not fully explained, how they're able to 'upgrade' and annex nearby provinces. I noticed the AI player built one near the player capital right at the end of the latest stream(ai doing that is a pet peeve of mine in turn based strategy games), but it was still mostly obstructed by the fog.
Second on the list is Sieges. I suspect I get the gist of it(spending turns to chunk through a 'health bar' before finally getting to do battle), and think putting it together with the above would probably make for a good dev diary - "Sieges & Forts".
Otherwise, I think I'd like to see a devblog "Diplomacy & Victory Conditions" around the time they let the influencers loose at the end of next month. (Hoping there are multiplayer games scheduled for the influencers to showcase the chaos across different levels of series experience.)