Alright folks, here's our tentative idea so far for the series of one-shot adventures, still rough around the edges but we're working on it. :^) Some input and participation from other DMs would also be appreciated!
A hobgoblin rebel has deserted after the Ulruzian rebellion has fallen apart, and brought with him a goblin shaman as his slave, as well as a contingent of lesser goblin footsoldiers. He has come to the frontiers of Mittendien, finding a new fortress being built on the former site of Tillich, which would serve him well as a stronghold. Since it's still in the early stages of construction, it would be easy to penetrate and take over, while potential goblin allies may be recruited from the nearby wilderness. The hobgoblin will probably want to disrupt the area first to soften up the opposition and distract them, so he can more easily storm the fort. He also plans to recruit allies anonymously to help in the effort, maybe gathering information first and then making some surgical strikes or diversions....
However, an evil druid in the area has also taken notice of the fortress being built on Tillich's ashes, and doesn't wish to see civilization take over the wilderness that they he/she was just beginning to renew after Tillich's "timely" downfall and razing. The evil druid has allies of their own, and will seek out further allies to destroy the fort-in-construction, likely attracting them through his/her initial actions against the fortress and its builders. The druid will use any resources at their disposal to drive off the workers, end construction on the fortress, and demolish what's already finished, sending beasts and men alike to assail and harass the Mittendieners.
Meanwhile, the Mittendieners will be trying to make further progress in building the new fortress, and will try to make the place defensible even quicker once the raids begin..... They'll have to work hard to fend off the assaults from both factions, and not to mention the assaults of Player Character parties aiding the attackers. The final adventure or two will involve storming the fortress and fighting through a dungeon-crawl amidst the partially-constructed passages, maybe with the PC groups competing, the fort likely guarded by traps and soldiers of whatever faction controls the place at the time......
There will likely be two PC groups of 5 characters each, at least that's what we're planning. We're expecting about 10 game sessions, one per week. For adventure ideas, we'll likely use some of the ideas Memnus posted for Mittendien, though modified. So far, Entropy is working on the evil druid and their faction, while Emiricol is working on the hobgoblin and his forces. I (Arkhandus) will work on the fortress of Tillich and the surrounding workman village, hopefully with equal help from Memnus.

I'm working on a layout for "Tillich II" and the fortress for now... We will have another brainstorming session on Monday at 10 pm Eastern (9 Central, 8 Mountain, 7 Pacific). We'd like to see other DMs there to help out too! We could still use help with the setup and/or running the sessions!