After digging more in the code I'm afraid the problem I encountered is only a small drop of water in the ocean and that the code, as is, can't support any serious modding.
It's clearly not done, at start, to be modded, that is comprehensible because such a succes of the game was not really expected :)
But what is breaking any modding is the fact that the basic rules of pro dev are not expected and a lot of work on the basic code is needed before anything :
- outsourcing of all hardcoded datas to permit them to be modified, eventually by more than one mod
- separation of the 3 classical tiers (datas, process, UI) and splitting in functions that contains each one functionality (the end_day code is a good example of a 'every is done here' code)
For now the only thing that modders can produce are overhauls of the game, each broking any other mod and all broken by any update of the vanilla code and, even if I have a lot of time to spend on a game I love, I've not enough informations on the next waited version to launch such a project so I think I'll stop trying modding and only produce an overhauled version for my own usage.