To me, the best kind of challenges are the ones which require lateral thinking and unexpected solutions, rather than just a lot of busy-work…
with that in mind, I propose this challenge:
Build a city on the higher levels of a mountain (whether it’s a mountain in the Darkmoor Forest biome or the clay mountains in the Desert biome is up to you), and then carve away the rock below your city to leave floating “islands” of rock holding the building up. Use bridges to connect the city together, and have a staircase leading to the ground…
and make the whole thing look “credible” – that is, make it look as though the flying rocks were there first and the hearthlings built upon them; and make the staircase connecting it all to the ground climb up in a realistic way between the rocks on different levels rather than being a single span from the top rock to the ground.
Building a floating city is one thing; but the challenge here is doing it in such a way that you can get to the rock underneath in order to carve it out. You’ll also have some interesting work of making those floating rocks look natural. Oh, and naturally there will be an extra layer of difficulty when you move everyone up there where food can’t grow – you’ll need to get creative about how you provide the food for those hearthlings who live up top. Will they have to walk down to the ground to farm and gather their meals? Can you keep livestock on some of the lower rocks, or maybe even organise some crops up there? Or will it come down to buying all the food you can from traders, while selling other goods to pay for it?
This challenge should be a great stress-test for both the physical limits of the game, and also the different options to keep your hearthlings fed and happy.