In Master of Orion, if your ships move through a nebula field, their speed are reduced to 1 parsec per turn. This adds some minor tactics to the game, but don't really impact it. At most, it gives you or them opportunity to gather their ships for defensive purposes, but other than that, it does nothing really to affect the game play. I think if you fight in nebula field, it also renders your shields ineffective (not sure if that's in MoO 1 or 2, but I think it's 2)
I've thought on the topic of nebulaes for a while now, trying to figure out how to add "splashes" of nebulaes to the galaxy. While looking at noise functions, it occured to me, why not have the entire galaxy covered in one big nebula field, with varying densities in each grid cell? Then if your ships can move "20 units", and one cell have 10 density, and another two have 5 density, it means you can move 3 grid cells. If one cell have 20 density, you move one cell, and so forth. If the density exceeds your movement points, it'd take more than one turn to move one cell.
The pathfinding algorithm will find the quickest path, but with this new feature, it won't always be a straight path.
This idea led me to another idea, having different stars be "seeds" for the nebula field algorithm, so the nebula will look logical and "fits" in the galaxy screen better than random noise function. With this idea, I thought of black holes, and their roles in MoO 2/3. In MoO 2, it does nothing except block direct paths. In MoO 3, it does nothing at all.
I thought, how about having black holes be the maximum seed for the nebula? They will have 100 density, and pull up the density in surrounding fields, causing massive travel adjustments. So after all the stars are generated, a certain percentage of them are converted to black holes (later I will add an UI element that allows the user to modify this percentage). My art for black holes are crappy, it will be replaced by a better art later on, so don't worry about that. Now, with all those ideas implemented, the galaxy starts to look like a nice galaxy instead of boring black background. (I also don't have art for nebula, that will change later on)
Here's a screenshot. Note, the noise function still need tweaking, as there's square artifacts (you can see squares easily, and I don't mean the squares in the grid cell art, but square fields of density)