> 100% agree. A popular "bring your own engine" editor would be awesome and would make a lot of sense (just not a lot of 'business sense').

This makes no sense at all to me. Part of what makes different game engines interesting is that they have different ways of modeling the world. How can you make an editor that's agnostic to the model? What's it even editing?

Unless your notion of 'game engine' is restricted to a subset that happen to use a very specific model. Like for example...2D tile-based platformers with finite maps.