The Poles would be fun for me, since I'm half Polish. Also after finishing the HRE campaign, it was a challenge, especially in the beginning trying to decide what to keep, and what to concede at times.
Of course, what's challenging also depends on your style of play. I actually found the Danes pretty easy, and I strolled along for a long time with a small empire.
I'm still thinking about it, probably take a MTW break while I take care of things at home and work.
Bookmarks