@pineappledan
Thanks for taking this into consideration. I've always felt that the true historical Great Powers (Germany, England, France, etc...) all had some of the least interesting civ designs (in both vanilla and VP), so I'm happy to see that people seem open to improving them. I'll try to address each of your points/questions below.
Krupp Gun/Cannon:
1. As you said, it is odd that both German UU's come on the same unit line. Having the second German UU be an artillery unit synergizes very well with having strong melee infantry do to their earlier UU. Making that cannon particularly good against military units further synergizes with a melee infantry unit that is particularly good against cities.
2. The Krupp Gun/Cannon does in part represent post-unification Germany, as the cannons were produced starting just before German unification, and Krupp-produced cannons were most definitely used by the German army during the First and Second World Wars.
3. Artillery UU's are some of the most uncommon UU's, while there are many melee infantry UU's, in both base VP and with 4UC. Thus, an artillery UU is just inherently more interesting and flavourful.
4. As for art, what about this:
.
It is the "quick fire howitizer" from Janboruta's Bulgaria mod and looks a lot like:
https://en.wikipedia.org/wiki/7.7_cm_FK_96_n.A. Of course, it does look a bit more modern than it should, but I don't think that's too big of a problem
Steel Mill:
1. I totally agree that the name is a bit stale. Maybe it could be called "Stahlwerk" (just steel mill in German), or "Blast Furnace"
2. While a factory replacement does come late, I believe that Germany should be a civ that is quite average, maybe even a bit weak, in the early to mid game and then drastically takes off in late renaissance/early industrial. This is fitting to the historical reality of the German people/nation/state, and presents a unique challenge and playstyle players will have to adapt to and optimize.
3. I agree with you that late UB's can be less fun to play with, but I think that is because many of those which already exist are just not interesting or not powerful enough. A unique factory that is truly much stronger than a base factory and does some unique things (such as boosting mines, tile construction, and providing iron, though it could of course do much more than just that) I don't think would be too uninteresting.
4. While it may highlight a similar thing to the Krupp Cannon, I don't think that is necessarily a bad thing. Between 1800 and 1900, Germany probably changed more than any other country ever has within a similar span of time, in terms of politics and economics combined. One of the driving forces behind that monumental change was industrialization, specifically steel and railroads. The only country affected more quickly and profoundly by the industrial revolution was England, which is represented in 4UC by the steam mill. Thus, I think it is fitting that the industrial revolution should feature prominently in Germany's kit, as much of their national identity since the middle of the 19th century has revolved around industry and industrial production.
5. Additionally, I really like the synergy between the Hanse and the Landsknecht (more gold helps you buy more landsknechts), and I think a factory replacement and a field gun replacement would synergize very well together as well.
Prussia:
1. This is easily my favorite civ I've played on VP, and I am well acquainted with them. I think that the changes I'm proposing would actually further differentiate the two civs, mostly because this would mean they no longer share the Teutonic Order (which fits much better with the rest of Prussia's kit anyway), and the Krupp Cannon, while a bit Prussian, is still a hallmark of post-unification Germany.
Beer Hall:
1. I would love to see what you would want this building to be like.
2. My concerns with a Beer Hall are that we already have many unique culture buildings, with two buildings that are very similar to a beer hall (Celidh hall and Coffee House), and there is also the tavern building. I suppose a unique tavern could be interesting.
3. However, I think your desire to have a Bavarian part of the kit is a bit odd, as you said yourself that Germany's kit should reflect a post-unification Germany. Why replace a pre-unification part of their kit (the Teutonic Order) with a UB that isn't post-unification?