Ooh, I like that idea... You could have bizarrely-shaped "rivers" running throughout the astral sea, turning and twisting and splitting in three dimensions, but you only travel along the surface of them so in that sense it's two-dimensional travel.
Also opens up the possibility of paths and routes which you have to find, can be blocked, etc. e.g. "Those damned Neogi have blockades setup on all the astral rivers to Krynn!" or "We'll need to hire someone who knows the way to the body of the ancient god."
One thing I've never liked about space is that, realistically, the best way to get anywhere it a straight line. There are of course some ways around that, particularly on very short trips, but they usually feel very forced.