Supposing a character ends up in the Astral Plane, according to the DMG you can find one of these pools which (if you find the right one) will return you to the Material Plane. But there isn't any detail on what place you return to. So, do you return to a random spot? Do you return to the place you left the Material Plane? Or are there one or more nexuses where beings will appear? For that matter, supposing you left from Toril, could you end up on, say, Eberron?
As a point of clarity, there is only one Material Plane in 5e.
The Player's Handbook clarifies with regards to the Planes of Existence:
it encompasses every world where GMs run their adventures, all within the relatively mundane ... Material Plane
This implies that there is one plane wherein all the world's such as Eberron and Toril exist. There are no direct rules on how to travel between these world's but almost every planar spell is not limited with respect to a specific world as long as you have information regarding that world (i.e. a sigil sequence for a teleportation circle).
In conclusion, any planar magic that lets you specify a location can allow you to travel to any world in the Material Plane (as long as the Material Plane is a legal target). If the GM selects the location, they can choose in the same way.
This related topic explains about a similar question that touches the points that are relevant here.
You return wherever the DM wants. Technically, yes, you can leave Toril and end up in Eberron. However, as you said before, you found the right pool, the subjectively right for you, is your own Material Plane, then you'll appear somewhere there where the GM put you unless your DM has a plan for it or is just a psycho (Who knows?), you should be back to your native plane.