Having said that, and having played AoW4 for 277 hours this year, the choices the game lets you make when creating a realm are not very detailed. There are 9 basic geographies, modified by 7 clime traits, like "desert realm" or "endless fields". But there aren't any sliders anywhere that let you for example modify the percentage of mountains on those endless fields, or how much space underground is unusable due to lava.

I don't know if a mod that adds such sliders would even be possible, as adding new menu elements seems difficult to me. But I do know that a lot of the parameters that I would like to change can be changed by a mod. I just need to write the mod myself. For example I wanted to try the Reavers new culture again, but on a map with more free cities around. And while there are realm traits to get fewer or no free cities, there isn't one available that gives you more of them. But if you look in the mod-writing resource editor, there is a setting for "dwellings" that can be set to "none", "few", "normal", or "many". The setting exists for various map types and clime traits, so I could simply modify the clime trait that I wanted to take anyway to add the "more free cities" feature to it. For somebody more talented than me, there is probably a way to create a brand new realm trait "More Free Cities" as a mod; me, I can only modify existing stuff, not add new things with a mod.

Anyway, by simply writing a mod for myself for just the next game I want to play, I now have even more possibilities to modify the random map generator to produce realms to my liking. At the end of the game I simply delete the mod again, or rewrite it for the next game.