Seriously though, "we all" want MineCraft, you have an engine that could support it, you are looking for an idea.

So, I mean, why not MineCraft ?

Of course, you probably won't make the whole game in one week, but nothing prevents you from first making a world editor where the player would be flying and would dispose of infinite blocks (kind of like the Creative mode) to place anywhere. This would already be enough for people to have fun, see how they shared their creations even if there was no goal in nCraft. And then, if you feel like it, you could add player gravity. Then if you still feel like it, you could add mobs. Etc.

Of course, I don't want to tell you what to do, but once again, people want MineCraft, you have an engine that could support it and you are looking for an idea

Ray-box would be the best IMO ; since everything in Minecraft can be leveled down to a single bounding box without losing precision, it could be good to have specific code for that.

This is too specific to be in nGL ... and Minecraft with raycasting ? -___-

Sorry, I meant raycasting for block picking. Not for rendering.

Wow, a Minecraft clone using this engine would rule! How does it handle a 512x512x64 map?

I noticed the edges of the polygons were shimmering. Is that because you are using integer coordinates for drawing?

I'm using fixed-point (1/256 accuracy) so it could be.
But I assume the things you saw were caused by a mixture of my .gif recorder and normal aliasing.

Edit:

Quote

Wow, a Minecraft clone using this engine would rule!  How does it handle a 512x512x64 map?

Not at all, I tried it. 16*16 chunks of 16*16*16 blocks = 1 FPS...

4*4 chunks is the maximum usable size (~8fps), too many triangles

How do you render them? If you render every block individually, you waste a lot of time drawing invisible parts. Even a computer can't handle this. You need to generate meshes which contain only the sides of the block  that are exposed to air (or to any non-solid block if you include any). Then you should notice a huge performance increase. (The real minecraft does this)
If you already do this, then you may want to look into other ways of hidden surface removal. I think you mentionned earlier that you use a Z-buffer? For a world made out of blocks, this isn't really nessicary, as they are very easy to Z-order. If you plan on including mobs as well, it gets a little  bit harder, but it should be possible as well. This should also increase the performance because it cuts down on per-pixel operations. (The real minecraft doesn't do this, but on a computer, you have graphics hardware that is designed to do per-pixel calculations efficiently, which the nspire doesn't have)
If you already do both of them, then I'm out of ideas to increase the performance.