Seeing China got a brand new UU give me the confidence of we we Germany can get rid of the boring Panzer UU too!
I think the probability is quite high, if Barbarossa is the german leader, we might get a special german medieval unit and if it's true that most Civs wont have 2 UU, the germans might get another Unique. Maybe the Panzer might return with some other german leader.
But back to china. I am not just seeing the possibility to turtle with china, the Great Wall might be even used in offenses. Until now it is quite unclear if you will have enough ressources or time to invest in massive builder training to support attacks and we even dont know how likely it will be to capture enemy builders.
But if you can support with china enough builders additional to your army, you could conquer an enemy city and directly forty your frontline to the remaining enemy, until you have crushed his armies. If the AI is still a bit same like in Civ 5, you might just have to wait for it to come you in the case of a war, so you can get full advantage of the wall.
And when you captured a city, you might even use the new parts of the wall to pay your army. It is still uncertain how good you can get gold in the early game, until now only coasts tiles and special ressources might give gold (and the commercial district). It is good that coast tiles give gold again, rivers dont seem to give them directly (but give boni to the commercial district). We dont know how fast you get that district, we dont know how fast you get your trade route or more importantly more than one.
In my opinion the change in BNW to get rid of gold from rivers and coasts was one of the main factors which hurt early aggressive expansion and conquests because you had to look for other ways to pay your army.
If china gets the wall quite early and the terrain yield still exists if the wall is build, it can help to maintain a bigger army, if you dont get fast enough your trade routes, commercial districts or you cant really use a coast city, which might be the new commercial hubs in Civ 6 especially if there is additionally a river for the commercial district.
Sure, there are still many unknown factors, religion for example, all ressources and their yields. And how likely it will be if you follow the military focused part of the tech tree, if you even get the options to increase your gold income. There will be probably civics which support an aggressive playstyle which give gold f.e. for killing units.
But even if plundering gives you gold, you wont probably use the offense wall spam because you might probably use your builders to get the infrastructure running again. But to be honest, when I wanted to conquer a city in the past I tried to avoid unnecessary pillaging.
Ok, just my 2 cents ...