About "readable format", I suggest that mainly because it's auto-documented, I mean I just have to look at the file to see what the mod does change. Else we'll have to rely on documentation files, and we all know how unreliable they are, whenever they exist
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If the GUI is able to display data as well as create it it somewhat alleviate the problem, but still I don't think it'll be very friendly if a mod changes *many* things.
Another point : did you succeed in editing Castles values ? Those, along with themes costs/preferences, are IMHO the main unmoddable unbalanced features (my MP games show 80% WT, some Castles-Wiz Towers, no Mnt Fort, Fort City etc..)