I have some questions about one-way portals:
1. What kinds of creatures might they bring into the world? Will they be relatively normal, or completely alien?
2. Will megabeasts come out of the portals?
3. Will the portals bring items into the world? And if so, what kind?
4. What realms are these portals connected to? Are they afterlifes, or different dimensions entirely?
5. Will the player be able to open them in Fortress mode?
6. How will these portals affect magic and creation myths?
1. A portal may connect point in the same world, different world, and different "dimensions/realms", so anything from completely mundane to completely bizarre is possible. What will be implemented in the first pass (where one dimensional ones are implemented) probably depends on what Toady comes up with at the time, i.e. he probably doesn't have anything fixed yet.
2. Quite possible, and it may also be a god/eldritch horror that's tricked mages into releasing it into the world...
3. Some kind of item and resource yielding portals have been discussed in the past, I think. What you may get depends on where the other end of the portal is (and what's implemented in the first pass)...
4. See 1.
5. Yes, eventually. Whether that will be implemented in the first pass or not remains to be seen, but it would probably be implemented early.
6. It would presumably be the other way around, i.e. the portal magic available and the external "planes" portal may connect to depends on what magic and "planes" myth gen comes up with.
Also note that one way portals aren't anything special in themselves, but rather a special case of portals that's easier to implement than the two way version (as that would require DF to handle two locations concurrently, which requires extra work. An early two way portal implementation might work similar to raiding does currently, though, i.e. squads are just sent through the portal to do their thing and then return with a report [or fail to return...]).