Ok, maybe I'm still too new to the game to post meaningful suggestions, but this is something I've been thinking about both from my own playing experience and reading community forts etc.
It seems like it is far too easy to make your fort impregnable to sieges and almost all other attackers just by digging a channel around it, or building a wall (OK, flying creatures might be different).
But what if goblins eventually learned that if they brought some picks with them, and also gained some way to break down constructed walls (goblin masons?) they could dig into your fortress from below?
Maybe catapults should also be able to knock down walls (not instantly but after a lot of hits), and sieging armies should sometimes bring them.
Finally, maybe there should be a few types of random mega-ish beasts that dig tunnels (some kind of giant worm?) that can also be a threat to an otherwise impervious fort.
Of course, I'm sure players would quickly come up with ways to pour water and/or magma onto any enemies that were attempting to dig into the fort.
I'm not sure if the thing about besieging armies will already be covered in the army arc, but if not I think it would be interesting and worth thinking about.