Something that I'd like to see:

The "Lost world" would look like a nether-style cave (stone, not netherrack) in the lower 128 blocks. Above that would be the top of the "normal" world. However, in addition to using the normal noise map to determine where to place stone above the nether bottom, it would also use the gaps in that noise map to determine where to carve air into the lower nether bottom.

In other words, the bottoms of the oceans, and/or possibly the rivers, or ravines/caves found up above, would cut down through the bedrock that normally seals off the nether style bottom, and cut through the giant walls of the nether. The result is that trying to ride a water current down a cave could drop you a hundred blocks into the nether bottom.

And, of course, lava seas for the bottom 32. Again, it would look like the giant nether caves, with stone instead of netherrack. Ores would be distributed down there (not really sure what frequency / distribution range / etc). And ... who knows what would be found down there (is there a dinosaur mod? :-).

@keybounce You will be happy to know that this is possible already with the new terrain generator.

Fossil/Archeology mod http://www.minecraftforum.net/topic/446117-sspsmp132fossil-archeology-v69-p-4/ has a lot of dinosaurs that you can hatch from eggs after finding fossils and researching them. They however would not be able to live in the environment you are proposing they need plants and water to live/eat. The meat eating ones require other dinosaurs or vanilla animals to eat.

@keybounce You will be happy to know that this is possible already with the new terrain generator.

You do understand the difference between "It could be done, if a new mod were written", and "Yep, already in the codebase"?

@keybounce
Yes, Let me rephrase: The current codebase of ebxl4 allows you to configure the world in the way that you describe.