DragouN

Yes, the Twins would be another border besides the Neck, the North is almost as big as the rest of the other kingdoms and is underused, the unification of the 7 kingdoms on the one hand was good, as it reduced wars, he has to take advantage of this time and develop his lands and not conquer others. Dragons can still cause destruction, and Valyria is at its peak, after the doom he has more room to maneuver.

It's not worth it, they have no borders, some lands would be ok, like the Freys', but the rest? Not worth it. Harrenhal is a huge expense to maintain, developing the North is less problematic, if he goes out to conquer other lands it will draw a lot of attention, and it's better to take advantage now to acquire a lot of things.