What will portals do? Can they be two-way transports to other locations, or a thing that creates new creatures? Can demons emerge from them? Can they appear in fort mode and summon vermin? Can they be summoned manually? How? Lots more questions
According to what's been said so far, the initial version of portals will be one way, allowing critters to emerge from them into the DF world, and "demons" should definitely be among the non native critters that might emerge.
Later development will see two way portals, but a precondition for that is support for multiple concurrent locations being viewable and controllable in Fortress mode. When two way portals are implemented (which will probably happen after the first Myth & Magic arc, as the seams of that arc are extremely strained as it is), they would be able to support transit to another location in the DF world as well as to other "planes". Magic should allow the opening and closing of portals (eventually), using multiple magical methods (such as rituals, invading powerful critters, "divine" support, etc.). Presumably portals could be temporary or persistent.
Vermin summoning might actually be harder than regular critters, although the mechanism by which cavern vermin is released currently might be used to release vermin from the other side. Otherwise, if you want a vermin spawner, a magical artifact that does that is probably a better option than a full blown portal.