The gameplay is very boring at later stages for me.
I tried 1] to start near volcano 2] to start near evil ocean 3] to start being at war with humans, elves and goblins 4] to start in area with undead whales, undead giant eagles, undead elephants 5] to start in area with NO metals at all 6] to dig adamantine veins, avoid demons and\or fight with them 7] to avoid cheats like 'danger rooms' 8] to start in ice 9] to start on cursed ground 10] combining all features together. It's all the same at the later stages of the game. A king comes, years pass by, fortress is safe and calm.
Don't think that challenge like 'live as an elf' or 'no magma, no caverns' is a good Idea. It's something like walking to a distant city while having a car.
There are several universal recipes that can counter almost any problem I met in DF. Those recipes are walls, massive cage traps, bridges and an ability to prepare and keep indefinitely a lot of food.
I suggest to implement problems that cannot be countered so easily. Those are 1] new embark areas such as old graveyards (maybe non-humanoid graveyards such as well-known legendary elephant graveyards) or battlefields that sometimes spawn a real bunch of skeletons, zombies and especially ghosts (that are capable of passing through the walls and disturbing dwarves). Skeletons may spawn everywhere within several first layers of the soil. Dwarves should be able to find old bones while digging in those areas - a kind of sign. 2] creatures (something like worms) that can safely travel through stone and attack from almost everywhere. 3] invisible 'enemies': rabies (dogs\animals), plague (undead) and flu (migrants\sieges), fleas (too many animals), animals or crops disease (too many farming), industrial injures (dubbing workers), alcoholism (a player should not brew too much booze) and involvement of dwarves in that (a kind of disease as well), prepared food spoilage (not 'rot', should make a game really interesting), all that problems may be brought by migrants, undead, sieges or animals. Any kind of problems that can be solved only by an appropriate management, not by a weapon or item.