I've been pondering more on the idea of using physical rules in a universe to drive not just the gameplay, but also the character design, plot, character arcs, level design...
This requires a good balance. The physics twist has to be interesting enough to drive all that, but also simple to grasp.
Two bad examples of what I'm talking about are Fez and BoomBlox. Both are good games, but they aren't good examples of what I'm talking about.
BoomBlox has a simple-to-grasp physics assumption, but the assumption is not deep enough to drive the story and character design. It's basically just a physics game where you blow stuff up, and build stuff to blow up.
Fez has a physics assumption which is deep enough to drive the plot and characters and setting - and, to a large extent, it does just that. However, it is difficult to grasp and brain-hurty. Even the most minor application of the physical assumptions is a puzzle.
Good examples include the recent Mario games. Paper Mario makes the simple assumption that everyone's made out of paper. This basic assumption allows for a huge variety of results. Characters are flat, so they stack. They can fold. They can be blown away, get wet, get glued down - everything you've seen paper do. You can also extrapolate to different kinds of paper, such as cardboard. Combined with the rich supply of content from existing Mario games (goombas and princesses and so on), the world feels super rich and easy to grasp.
Mario Galaxy is another example of how to do it right. The physics assumption is that the universe is made out of little planets, each with its own gravity. Because we're familiar with this kind of "Little Prince" assumption, we can really understand the results pretty easily, and it gives the game a lot of excuses for fun islands of uniqueness.
Mario Sunshine is anotherother example of how to do it right. The physics assumption is that water and paint are the primary concerns, so most everything that happens revolves around the nature of water and paint.
Neither are as sharp as Paper Mario, which pushes further, easier.
A wasted example is Epic Mickey, which was theoretically based on the nature of toons. In practice, however, it was mostly just an ordinary action platformer.
Kingdom Hearts is a pseudo-example, where the nature of the world is "all those movies and games you've played". This is a very rich vein to mine, which is why the games are so popular, but it's a bit too linked with pop culture for my interests (or budget). I would prefer to come up with a different physics assumption besides "self insert crossover fanfic".
I've been thinking about how to create these.
It's tempting to just flat-out steal one. As long as you don't steal the IP associated with it, that's legal and not immoral. I could make a game about paper cut-outs. I could make a game about paint, or toons. (Although I don't think I could call them toons, because that word is trademarked if I remember correctly.)
I could also take an assumption that had promise but didn't take it very far. For example, Portal. Portal's physics puzzles are excellent, but if you start over and assume that portals are common, you can start to build a world where the whole world - all the characters and settings and plots - involve the ability to pass through portals. As an easy example, snakelike creatures would be common, with the ability to pass their heads through portals while leaving the bulk of their body in their lair.
But instead of simply copying and enhancing, I'd like to think about the nature of these kinds of worlds.
Perhaps the core measure of this technique is how transparently the assumption becomes part of daily life in the game. The assumption has to be something that all the locals just take for granted.
But, on the other hand, it also has to be something that human players can easily comprehend. The idea isn't to make reality more complex, just to make it different.
A good example of that is Paper Mario - everything's made of paper. It's just taken for granted - toads hide by slipping under doormats or stacking on top of each other. People take care not to get crumpled or wet. It's part of everyday life. When a toad says something like "watch out! Don't let me blow away!" the player instantly understands the concern and what's going on.
On the other hand, if we were to ramp up Portal, we would need to make the ability to travel huge distances instantly part of everyday life. This would probably be too hard for the player to instantly understand, because the human mind is very much about object permanence. It takes a lot of thought for us to comprehend the idea that space folds like a crazy straw. We can't hold it in our heads very well, and there's not usually an intuitive leap. This is made worse because most of the time, the other end of the portal will be in an unclear location: not only is it unintuitive, but we're always working with hidden information.
I think the key is to pick something that players are familiar with, and apply it in a way that they are able to understand even though they've never thought of it before. Sort of like the movies "Toys" and "Cars" - everyone knows what cars and toys are, so the different personalities and plots and characters are really easy to understand.
Of course, easier said than done.