I don't agree with adding adamantine to none-mountain maps, but I do think there should be a load of non-mountain features. And, also, they don't NEED to be adamantine. There are better things than adamantine.
Maybe there could be underground tombs hidden in jungles and deserts, with an undead lord and a few undead soldiers defending the tomb of an ancient king, complete with an artefact-level object located down their?
A demon living in a hidden shirne, with a random group of humans, goblins, dwarves, kobolds and elves all worshipping him? The shrine could have a horde of gold objects and a few eleite guards weilding masterpiece steel weapons?
Maybe if mega- and semi-mega- beast caves were improved into something more impressive than a sprawling network of 1-tile thick caves then they would be a good feature to settle on top of. If these caves were made into something more impressive, like a large cavern filled with gold crafts in the case of dragons etc then these would be a good challenge as a site. You would have trouble living next door to a dragon, but when you did finally kill it you would have a pile of gold crafts and a great new expansion to your fortress.
I certainly think that the undead need to make a come-back. They were nice to have around and I think bringing them back would add far more flavour to the evil area's of the world. I eman, what is better than a stepped pyramid in the middle of a jungle, swarming with undead soldiers and full of valuable crafts?