DF is a true civilisation simulator - most civilisations collapse for internal reasons, the same is true in DF. Sure, external pressures like goblin hordes might push a teetering fortress over the edge into chaos and fire but only if the fortress is at that point due to internal troubles already - tantrum spirals, insane demands, a psychotic Hammerer, catastophic engineering failures and suchlike.
Limit or eliminate automatic trap useage and marksdwarves. Don't brick yourselves in either, dwarves have no reason to fear lesser racers and such have no need for defensible entrances. Impressive entrances, yes, but none of this 6-steel-doors-in-a-row business!
Wildlife in general isn't that dangerous if you are careful no matter what. So the key is not to be excessively careful - build structures out in the open, lets dwarves wander about and protect them your your military rather than by keeping them behind a massive wall. It won't suddenly make it "hard" but it will give you more to do whilst increasing the risk and thus the fun in the process - every random champion impaled by a raging unicorn is one less to fight the goblins and one more to build a nice big monument to. Every goblin ambush that happens in the middle of your "city" suddenly becomes something urgent to focus on rather than a minor event to note only because the increased hauling will delay projects slightly. And so on.