What I'm most interested in are dynamic interfaces- interfaces which change depending on what's going on. This would be easiest to pull off (and most effective) on the DS, so let's say this is on the DS. The idea I thought I'd post a minute ago would have been a hybrid interface, with movement on the D-pad. But now that I'm writing, I figure, why not go the whole way? So everything's done by the bottom screen.
On the top screen is the 3D gameworld, shown in cinematic camera angles that turn around as necessary. (The player has no direct control over the camera.) The bottom screen is covered with buttons for contextual actions ("Exit, Talk to salesman, Hop Up and Down while Singing 'Thus Spoke Zarathustra'") pictures of key objects in the vicinity (a screwdriver, a stereo, a lit dynamite stick, a purple cat hopping up and down while meowing "Thus Spoke Zarathustra"), and/or a few (as few as the designers can get away with) consistent buttons like one for opening the inventory. Pressing on one of the pictures moves the camera to a better position to see it (like zoomed in real close while the cameraman jumps up and down to the tune of "Thus Spoke Zarathustra"), and maybe some new buttons will appear (For the stick of dynamite: "Hit with hammer, Stuff in Stereo, Eat, Ignore, Run Like Heck, Go Back") and maybe even a little textbox will pop up with the player character's innermost thoughts! (For the stick of dynamite: "Hmmm.... what is this? I've never seen a thing like this before... Nosiree, I've never seen anything like it... I have no idea what this is..." and a button marked "Ponder Further")
See, the beauty of what I just said is that I haven't really said much of anything. There's lots that the designer could put in, but nothing that needs to go in! Since this is a "dynamic interface", the designers get to put in whatever is most dramatic/funny/effective/reminiscent of "Thus Spoke Zarathustra" for whatever's going on. If you're outside, then the designers could put in a top-down map, or buttons for all the buildings nearby, or a signpost, or a little rhythm mini-game of skipping forward to the tune of..never mind. And just think of the amazing possibilities there could be with an ever-changing interface!
For my first example, let's say the designers (like any good, righteous, "Thus Spoke Zarathustra"-fearing adventure designers) are passionately in love with pixel-hunting. They wouldn't mark out the objects- oh no!- Then they wouldn't be able to give you the fun of finding them yourself! So instead, they'd put buttons (or pictures) for each of the locations objects might be hidden in (Top Shelf, Middle Shelf, Bottom Shelf, Left Shelf, Right Shelf, Floor next to Shelf, Underneath the Shelves, Behind the Shelves, On the Ceiling above Shelf, Inside the Unscrewed Knob on the Left Side of the Middle Shelf, On Top of Shelves Examined Under Microscope), each one turning the bottom screen into a 2D representation of the area for you to enjoy yourself pixel-hunting in. Now, if the player has been searching for the brilliant hiding spot (the third black dot to the left on the front of the bottom shelf) for a reasonable amount of time (seven hours) and is still too pathetic to find it, the merciful designer can have the game start to eliminate the buttons that lead nowhere (or, if he feels like having some fun, the Bottom Shelf button), to point the player in the right direction.
My second example assumes that the designers are godless evil simpletons who shamelessly want to spell out how to push the story along. Some of the items in a room will be important to the story and are obviously important to the character (say, his pink bunny slippers. This character is obsessed with his pink bunny slippers.), while other items are only there to flesh out the gameworld a bit more. The buttons (or pictures) for the most crucial items could be bigger or placed to attract attention on the bottom screen, so that the player (if he is the sort of mindless bloodsucking drone that these evil designers worship) can play through the game quickly if he so chooses. (Bah!- free will.) Or if there was an item which the player ignored before, but now has become crucial to the plot, it could get bigger to attract attention. Or a button could start out big, but get very small once the player has already seen it so it shouldn't distract.
Similarly, some characters are extremely important (the hero's girlfriend), while others are not (the hero's wife). The button to go talk to the girlfriend (or join her in hopping up and down to the tune of Thus Spoke Zarathustra) should be bigger than the wife's button, so that the player always understands what the character's priorities are like.
Let's say our hero is having a conversation with his wife (dialogue would also, obviously, be handled on the bottom screen), and really should be telling her that he sort-of-accidentally allowed her beloved purple cat to be blown up. This would be a pretty big button, since it's weighing heavily on his mind. Naturally, the player will try to push it, but whoops!- the button hopped over to another part of the bottom screen. Try to push it again, and again it hops as our hero puts off the inevitable. Eventually the button may try to hide under some other "excuse" buttons, or jump to the top screen where you can't reach it, or something like that. Now that's drama!
(When talking with his girlfriend, half the dialogue choices would be truly pathetic and half almost-intelligent, and the buttons would be sort of wavy and shake around, so that it's hard to press on the right one.)
If you're inspecting the scene of a crime, the game should move slowly to let you figure things out, take notes (on the handy-dandy bottom screen), etc. And by "slowly" I mean "slowly". It shouldn't be rushing you onwards or reminding you that your wife is outside carrying a club. The trouble is, the player is the one in control of the progression just as much as the designer. A dynamic interface can be used to encourage him to slow down. Let's say that since the beginning of the game the buttons have been hopping around, there have only been a (relatively) few buttons, and those buttons were often pretty big. Now you walk into the scene of the crime, and suddenly you've got no movement controls, no hide-from-wife controls, but twelve pictures of pieces of evidence (or red herrings) all given equal space in a four-by-three grid. In addition, everything is given narration by the character in a textbox at the bottom of the screen. As soon as the player sees all this, he understands that he's meant to go slowly.
On the other hand, let's say in the next scene he finds out some urgent news. (His wife is driving home to rip his fluffy pink bunny slippers and smash his record of Thus Spoke Zarathustra!) Let's say the designer is staying away from timed sequences- how can he indicate the urgency and speed up the game? Simple- he takes away all buttons but the "Run Like Crazy Back Home" button, and has that button take up two-thirds of the screen, hop up and down urgently and flash. (If that doesn't get the player's attention, maybe it could have an obnoxious sound effect like a siren combined with a French Horn.) As he runs back, he'll go through many areas he's been to before, but the buttons will (at least mostly) be missing: What difference does that third black dot to the left matter when the music (and the fluffiness) is in jeopardy?! It doesn't matter that there's no time limit- the player will run home (because he doesn't have any other choice) and won't really feel too cheated. (Because he understands that buttons can appear and disappear at random, and because -overall- this isn't too long a scene.)
These are two extremes, you understand- I'm not saying it would ever have to be so exaggerated.
Symbolism and Other Gimmicks
When you can play around with the layout of the interface at will, you can do all sorts of nifty stuff. Have you ever read David Mack's comic book Kabuki? No, I didn't think so. Oh well- that was a good example of the style I'm talking about. How about Bill Willingham's Fables?- That did stuff like that occasionally... Never mind. Okay, let's say you're in a garden, and you want to hit the player over the head with the word "flower" because this is some deep artistic nonsense. So you could arrange the buttons on the bottom screen so that the layout looks like a flower! Pretty cool, huh?
Practicality, practicality. Always you yell about practicality. Okay,- I'll give you practicality. Let's say there's an unresolved mystery in the story- it's just sort of there. But you want to tell players the answer to the mystery, and you just want to hint it. So in some important scene, you've got one picture in the center, and all the others around it. And why?- the player wonders. Why, it's because it's a clue, you nitwit! And he sees this clue, and then he understands the mystery. What do you mean, "pointless"? You're too picky.
Anyway, dynamic interfaces are a whole new language. I'm sure I've barely scratched the surface of all the possibilities that would be opened up here. You can probably think of more yourself. Something to think about...