A sprite editor would be nice, but it would be a sizable development project if you want something fully usable (so it will definitely not be in the upcoming 3.1 release).
Even GameMaker's sprite editor was rather basic for years before the purchase by YoYoGames (which is back when I used it). Right now, you can do some object creation using 2D polygons and shaders, the scene-based structure of Godot will do a good job of keeping everything together.