Sadly my German is way too poor to write in it (fortunately Google Translate does its job well enough for me to understand what you wrote in German) so I'll stick to English. I wasn't sure if I should write here or at AnnoZone to ask for advice regards modding but the problem solved itself because I cannot complete registration on AnnoZone (the site doesn't consider Venice nor Venedig the correct answer to the security question) and this is the official forum for the only actively developed mod I know.
I am planning a mod for the 10th anniversary of Anno 1404 release which adds an option to build cities in Italian flavour in addition to the usual Hanseatic and Arabic themes and rebalances some aspects (not as much as the IAAM because I wouldn't like to kill the original ideas). This comes with some basic questions regarding what can be modded and what cannot which I wasn't able to answer myself with the help of Google Translate. I'll add answers if I find them out myself.
Q: Is there a working .gr2 exporter for Anno 1404 so I can modify original building and ship hull models and use their revamped versions?
A: Granny3D 22.214.171.124 exporter plugins work correctly on Windows 10 with 3DS Max 2008 when the export is done from 32bit version of 3DS Max. The problem is there is no direct GR2 import plugin so one has to use GrnReader98 losing some important information about the model and animation and Anno 1404 uses Granny3D 126.96.36.199 so some of the newer features cannot be utilised with the exporter. I haven't tested adding new animations yet but the plugin I found comes from an official release so it should be fully functional.
Q: Is there any chance to add animated oars to a ship?
A: I am still testing the ability to add 188.8.131.52 animations to the game. Stay tuned.
Q: Is it possible to add a new citizen class (with the portrait model being just a reskin of an e xisting one)?
Q: Is it possible to add a monument with Garibaldi being the one overseeing the works?
A: The answer seems to be: yes! It involves quite a lot of work to code the proper reactions and events but it is not impossible. I'll add a tutorial when I manage to add a fully functional one.
Q. Is it possible to add a new class of islands, specifically Mediterranean islands, which are spawned between Occident and Orient, and adjusting the fertility spawn rates (so they are the only ones with Grape fertility)
Q. Is it possible to change appearance of a building based on whether it's built on Occidental or Oriental island (streets and trees do that but technically they are not building)?
Q: Is it possible to add a new building menu panel after the Envoy one?
A: Yes and no. There is very little space left for another tab after the Envoy one but it is possible to add buttons switching between multiple building menus so one can have more than one set of tabs each hosting up to 6 tabs.
Q. Is it possible to make the quay walls provide street connection? I find it ridiculous that units can walk over them but they are not considered roads.
Also, a balance question: do you think it would be too much of a balance disaster to add an ability to construct houses with Citizen level as a base (Peasant level game doesn't last long and is extremely boring anyway)?
If you got this far I have to thank you for reading this long post