A few tips that I would give about design in general.

Make sure your classes only do one task. If you have to use the word 'and' to describe what a class does then you should split it up into multiple classes. Keep the parts as disconnected as possible. The code that does the drawing shouldn't depend on code for collision detection and vice versa.

I would only make an editor if it were to facilitate making a game that you are already working on. Don't try to make a generic game editor that is capable of making every type of game. The game editor should use as much code that makes sense from the actual game. This could be the drawing code, collision detection, ect. Also, don't add features to an editor unless you actually need it. You can spend a lot of time adding features to a game editor or game engine but if you don't have an immediate use for it then it is a bit of a waste.

Also, I would like to give out a warning against global or singleton managers. Adding global objects makes code less flexible and testable. Global objects are tempting because they make access to them really easy. It takes less work to plan out the code. In the project I am working on now I have made it a point to not have any singletons and it has been great. Before I would have a singleton resource manager, that way all loaded resources are cached in the same place. It seemed like a reasonable use for singletons. The project I am not doesn't do that. Instead I pass a resource manager wherever loading resources is needed.

Just the other day I realized how easy it would be to put all of my resources into a single file because of this. Instead of having all resources loaded from a non-changing global object. I can pass it different types of loaders. One loader takes a resource path and finds a file in a folder, or I can pass in a loader that loads the resource from a single combined file without making any change to the code that uses the resource loader. This is especially useful because the game I am working on is an HTML5 game letting me work with my resources organized nicely onto folders, then when I finish it, I can merge all resources into a single file letting them be loaded in a single request.

Some of my thoughts.