When the player defeats an Ender Dragon, an end gateway is created that teleports the player to the outer end islands. It would be nice if these portals also displayed their destination.

While it might make sense for consistency, I'm not convinced there's much practical use for it.
You'll usually look at end gateways from below, so all you'd see is the sky, and due to their size you can't even go through them.
I don't think its worth having to load all the end gateway destinations for that whenever you enter the end, especially after defeating the ender dragon multiple times.

I hadn't considered the issue of loading so many destinations at once, that makes sense. A possible solution is to only load the area once you get very close to the portal (within 30 blocks?), but I can see why this isn't a priority.