I think that the inclussion of the Panzer as a UU is almost inevitable since there are a very limited number of civs with potential modern UUs.
That being said, I am betting that since Barbarossa will be the leader, he will have the Teutonic Order as a Unique Leader Unit. Rather than be focused in calvary, Teutonic knights would perhaps recieve bonuses regarding melee combat, faith conversion, and the building of outposts.
As for other unique UUs, I would really love to see the German U-Boat as a unique early naval unit since that was definitely a game changer (sorry Zeppelin, you were iconic, but ultimately useless) or the Paris Gun, the very first industrial era superweapon (!) or their smaller (and more useful) cousins, the Big Bertha cannons!
And if they want to open the WWII can of worms, there are a ton of UU to draw from: the Messerschmitt Bf 109 aircraft, motorbike anti tank units riding BMW R75, and much, much more.