I have removed the post from the superuser to prevent cross-posting. I have read the article about it and couldn't agree more. This is something I will definitely keep in mind before asking my next question. Hopefully, you will still take some time to discuss this topic.
As per the link you provided, thank you! I have read them and I want to let you know, that I was already familiar with Picture Content Control (PCC). And if I understand correctly, what you suggest is to use PCC to only display an image of my "custom control"? Is using a PCC the "best" or the most "correct" way of doing this?
Like for example, if I want to add right-click functionality to my "custom control" and extend the context menu to have another context menu item. Another example would be double-clicking. Yeah, sure I can catch a double click event on the document and somehow find out if the correct PCC was double-clicked, but I would imagine being able to handle those events on my control so I would not be needing handling the double click on Word.Document class. Perhaps I misunderstand something, I am really not so familiar with the VSTO plugin development. I just want to find the best practice to be able to extend the functionality further through the years and prevent the situation where I will have to rewrite everything in a few years.