I didn't really get it until I downloaded it. Excellent job. I only played with it in a cursory manner so there's a lot don't know about.
The first I have to say is that's a whole lot of white. A dark mode would be nice. I don't really want to stare at that much white for long. It couldn't find my mods directory. I assume that's because it is in %AppDataLocal% rather than %AppData% which is set to roaming. If you're just checking relative to %AppData% then you need to add in checking in %AppDataLocal% if you don't find it in %AppData%. I see it creates a directory in %AppData% with a bunch of files in there. Particularly I don't know if that's files modified through using the application or if there's configuration files a user might have cause to edit directly. There's no installer so I'm not sure where I should put this. Particularly whether it's 64-bit program going in "Program Files" or 32-bit going in "Program Files (x86)". That isn't to say you need an installers, that's just nice to have, but that you need installation instructions absent an installer automating installation. If there's any configuration the user may need to adjust then that needs explained as well. The zip file contains a directory that contains the directory the application is in. That top level directory would be a good place of installation and configuration instructions.
That brings me to the general need for documentation. This is a bit more involved than notepad and involves an interface most are likely unfamiliar with. Some instruction on using the program would be good. Markdown (.md) is a good format. You can read it with or without a viewer. Originally, I thought the writer was just including notes on what they would do if they used a real editor. HTML is another choice. Generally not real readable without a viewer but everyone has a viewer. You can also use Markdown as a way to launch into HTML. You should answer the basic question of how do I get started. Just as an example when I start the app it tells me it can't find the mods directory, something about a JSON file or select a directory. I don't know why I'm doing that. I think surely I don't do development in the live mods directory. I might want to play the game before I get it to an executable state.
Overall, I don't get the project model. Just to me I want a project directory where all the files containing information I enter into your program resides. Not configuration for your app, but information specific to this mod I'm working on. That is, basically, my source if I use this app to generate my mod. I want to generate into a clean directory appropriate for moving into the mods directory. That let's me check it to be sure I generated what I intended, i.e. did I use the app properly. You seem to be geared towards a DLC type mod rather than a UI mod. That's fine, but I might want to use it to just generate a modinfo file. Once I think it might actually work I want to test it so move it into the mods directory. Basically development, quality assurance and live. Some documentation explaining how you envision that process would be nice. I can well understand dreading the whole documentation things which is why so many programs go undocumented. I just don't think you can do this level of complexity without it.
I really like the whole entity relationship diagram thing. I think that's quite innovative. I give you kudos for that. Particularly since all gameplay is done through the databases and those databases get quite complex. That said I can't help but feel you need some standard interface elements. Specifically a menu bar, tool bar and status bar. I can understand from an ascetic perspective not wanting those. This seems primed for a multi-document interface. These diagrams could get quite complex. Your screenshots shows how hard it is to capture everything in one document. A civilization might have custom builds and units. You might want to split those units out to separate documents. The common MDI is tabs that let you select documents. The tabs actually just control which window is visible. That window is sub to a panel and fills the panel. Some apps let you break a tab free and move it around as a window constrained by the panel, but not forced to fill the panel. Some are not constrained by the app window. Those seem generally be creative apps. They need all those menues and tool bars, but not right now and the user wants immersed in what they are doing. Often they have detachable palates so the user has access to the commands they need for editing. That would seem a good interface here. That isn't an arguement that you need to support multiple documents now, but that you can detach the diagram from the app window so you get that clean view when the user is immersed in the fine details. I'm not sure what you're using for development but it's generally a simple change You just make it a child of the desktop rather than your app. Otherwise it's indistinguishable to your program.
You do have a status bar. It just displays the x, y coordinates presumably because you can move the boxes around. It would be nice to have some additional information. Like maybe when an item is selected how many relationships does it have or how many items are there in the diagram. One thing in particular that would be nice is messages. Trying to figure out how to do things and I get messages that disappear never to be seen again. Something I can click on the status bar to pop up messages would be really nice so I can read them. Really nice would be a count of how many new messages since I opened it last. Like a basic SMS app. You might as well put the time in there as well since you can never have too many clocks. And the save status. It is there, you might as well use it.
Overall, excellent job. If nothing else I look forward to using to explore how things fit together. I am curious what you used to generate those diagrams, I got some data that would make for a much easier way to view.
