First off, Dalaran itself contains many portals. The new tower in the city’s center has a portal room called The Chamber of the Guardian with access to many locations — the Caverns of Time, Dalaran Crater, Karazhan, Wyrmrest Temple, and Shattrath City. Also, each faction’s side of the city will have a portal area as well, with portals to the various capital cities as well as that faction’s shrine in the Vale of Eternal Blossoms and all of its portals.
Essentially you’ll want to use Dalaran as your travel hub. If it doesn’t have a portal link to a place you want to go, it has a link to a place that links to a place you want to go. And since you’ll have a Dalaran hearthstone, you’re still free to set your own hearth anywhere you want in the world — you could set it to Burning Crusade Nagrand if you wanted — and if you have a Garrison Hearthstone, you can easily use it to get to your Garrison and hop over to Ashran to use the portals there.
Essentially, don’t worry about getting off of the Broken Isles. You’re most definitely not stuck there.