Actually I believe you answered my questiion. From what I understood there is a modding system for which you only need to put the mod in a directory to have OpenXcom use it. I guess, unless there is a plugin system of any kind this mods should only be the "ruleset modding". I wanted to do a few modifications and make them available without rebuilding the game, because I don't think they will be integrated in the core OpenXcom.
The modifications I have in mind for the moment are:
1 -
"your OCD is bigger than Mt. Everest".2 - Giving a chance to have an alien infiltration mission fail, by preventing enough UFOs of the last wave from succeeding. I think there will be a base percentage success of 0% and each UFO succeeding a mission will increase this percentage by 25% or 50% according to its size.
3 - Same thing with alien base missions.
4 - When an UFO is destroyed by base defenses, the retaliation mission ends.
I don't think those changes can be done with only ruleset modding.