I've been thinking. Here I talk about how rules are essentially gluing agency to content, and it's a very powerful concept - I've started up a new game to test a system of semi-rules, see if I circumvent the normal flaws that rules have by focusing on simply gluing agency to content in every conceivable way.
But I quickly realized something in the planning stage: it's not simply content and agency, at least not the way they are normally described. Simply pushing around images and concepts doesn't have any punch to it.
It's the nature of agency, I think. Agency isn't just the capability to change things: it requires a level of emotional investment. It basically requires them to care about the universe you've given them agency in, most especially over the specific characters you've given them agency over.
How to lure players into caring about your universe is a long post - I know, because I wrote it just now. Way too long. Suffice it to say that a big part of it is using the tiniest little interests of the player and hammering them, weaving them into content and agency and expanding them into a full-grown universe.
For example, every child is delighted the first time they realize they can move the little space ship around the screen. They quickly realize that if something runs into them, they don't get to move the space ship around any more. That's all it takes: the rest is simply using those tiny little entryways into their mind and turning them into a superhighway. It doesn't have to be gameplay: the classic RPG uses appealing cultural norms in much the same way.
It's not exactly gluing agency to content: it's getting the player to value the content. And it's just as important.