In games like Oblivion there is a thriving 3rd party mod/plugin world. You can get mods for practically every aspect of the game.
How could I write an android game so that I could support additional content written by 3rd parties (which they could charge for in the app store)?
Basically there’d have to be someplace that mods registered their existence and the game would have to check it and then somehow either load data from them (for purely data based add-ons) or execute them (for behavior changing add-ons).
To start with I think I’d mostly be interested in having add-ons which are data (it’d be safest), but I’d like to keep an eye toward executable add-ons.
In a sense you could have all your addons be data addons.
Consider if you wanted to add a new area in Oblivion. You could define a terrain dataset, some static objects as a dataset (trees, rocks, etc), connections as a dataset (doors to houses).
The easiest way is to make your engine just an engine, and the ‘game’ itself just an addon that you distribute with your engine. Your game is nothing more than a well tested, well thought out dataset.
It gets more difficult when you want to simulate something like a trading card game. Any special mechanics on cards would be limited by what calls you have available in your API. But, depending on how much you open up, the community at large will come up with excellent things!