Modability is simply how easily a game can modded. Some games are easily modded. They are designed to be modified by the players. Take Minecraft for example. Or any of Paradox’s games like Europa Universalis IV. Those games have active modding player communities. Minecraft has a series of well known APIs, and the game files for most of Paradox’s game are in easily modified json files. In my opinion if you own a game, you should be able to easily mod that game.
A central goal for my game will be to empower the player community to create any type of world they can imagine for a life simulation and role playing game. Yes, there will be obvious limitations within the framework of the game model. There will be the well known areas of game play: farming, fishing, community interaction and adventuring. It will be a sand box, but it will be an easily modified sand box.
A design goal for my game will be that nearly every aspect of the game can be easily modified by the player community. All data files will be in open formats such as png or json. The json files can be modified by anyone with a text editor. The map files can be loaded and modified in the openly available Tiled application. There will be no hidden APIs or data files that the player community cannot easily modify.