Great question! tbh it's more just I'm experimenting with what I think is a cool feature or not. Most of my games aren't really planned for, I just add a feature and if I like it (or don't like it) I either add it or remove it from the next version.
For example, from greater diplomacy 1, I thought Attrition would be annoying to deal with, and terrain would be annoying to have to think about when moving troops, so I removed those.
Although, in other cases, it might just be that I simply forgot to add them, or just because I felt too lazy to implement them. The research feature in greater diplomacy 2 was amazing, and I feel like I should have implemented that in the other versions as well. I just wasn't exactly thinking about it while making the other versions. The max factories feature was also great, since it would have forced a player to have to expand outwards at a certain point, instead of just spending the entire game focused on economy.
Originally this was going to be greater diplomacy 4, but I realized this version was so different from the others I decided to just call it "hex edition" instead. It was also very difficult to work with because I was adding in so many absurd features, and I felt like it was going nowhere so I decided to just scrap the project. But I thought if I just moved onto another project, I would have wasted all that effort on nothing, so I decided to just publish this anyways (with some decoration so it doesn't look completely unfinished).
For your concern about the games not being compatible with one another, that's probably just because the games are not designed to take save files from different versions. I could *theoretically* allow people to convert save files from one game type to another, but that would require me to manually correlate every single province from one map type to another. However, for hex edition, I don't think it would work. Hex edition uses many hexes as provinces, which can be changed by the player, while the other versions use set provinces, which cannot be changed. The variability with the hexes would make it pretty much impossible to map which hexes go where on a regular greater diplomacy map.
Don't worry though, in greater diplomacy 4 (yes I am working on it) I will add the features I like back into the game. There will be research, and there will be a cap on the maximum amount of factories you have. Since greater diplomacy 4 will be using manually set tiles, I could *maybe* add a system where you can convert greater diplomacy 3 saves into greater diplomacy 4 saves. If you have any other suggestions, please let me know and I will definitely consider it!