"Portal" is just a name. Teleportation is so anachronistoc that it's never going to happen.
So what if it is anachronistic? I would find teleportation highly useful and interesting in large maps. Reality can go take a flying fuck at a rolling doughnut, like Stephen King put it, as far as I'm concerned and as far as the game is fun and interesting.
I also think underground rivers were very beautiful and interesting and it's a pity that we lost them, together with the other features.
A few more ideas:
* Meteor strikes
- a giant ball of magma that creates a magma pool and fires all around when it hits the ground. Maybe one dwarf will have bad dreams and visions of doom prior to it's striking?
* Earthquakes that create a giant chasm. (and thereby multiple other problems.)
* Floods, river or sea rising 1 Z level or more.
* Magma / volcano activity
Magic ideas:
* Rivers could be magical in nature, having items that act as water sources and as sinks destroying any water they come in contact with. Creating water and destroying it.
Same with magma. Heck, you could probably make types of magma that cool in time and do not retain heat, thereby cooling into metals and other useful materials besides just obsidian. Your fortress could have an infinite metal source like this.
You could make mechanisms to activate them or at least move them around. Equipping your tower with infinite sources of magma/water/materials.
* Dimension travel with your fortress, just like with Doctor Who's Tardis.
Maybe a dimension of "Adamantine" full of demons? A dimension of poisonous forests, animals, etc... other possibilities to: Get a new start and environment to ruin with your existing "Dorf Tower".
Heck, even traveling and teleporting the fortress in the existing map would be cool. *WOWOOM* You've teleported next to the goblin tower.
* Flying dragon troops and bombers that will try to steal all our adamantine and impose a government to ensure we make no more Adamantine weapons! Weapons inspectors! *Sanctions* and bombings and siege to get a regime change if not accepted!
* Religious zealots and forces coming outside to demand we give up those among our fortress dwarves who are of particular *WRONG* faith for burning at stake.