On the code side of things, coming from someone who dipped a toe in RPG Maker a little, it's also possible to create events on the map that will self-trigger upon entering the area, check whether the variables you use have values and then create and assign them default values if necessary. So a form of self-check if someone loads a game from older save. It can be tedious to write it, and later update it when you progress the development, but once one is done you can simply copy it to other maps and it should work. More or less. Or run those checks where they are needed. But the specifics of it will largely depend on what addons you use (for example the one adding quests to the game menu).
It's not the perfect solution as it will extend the time you spend on the game without actually progressing development but it's one of the options.
