Kaiserreich, The Fire Rises (map might have to be during the action, and not at the start date of the HOI4 mod), Finno-Korean Hyperwar, Red World, war of the worlds and some of the other ones from greater diplomacy 3.
noted, I might add some of those if I feel like it.
also, if you're interested in adding some of the GD3 scenarios, did you know that GD4 is backwards compatible? If you have a GD3 save file you can load it into GD4 by pressing the "Load GD3 Save" button in the bottom right corner in the main menu. It's very basic and not very reliable, but it works.