Reforged uses the same engine as the original wc3 and they are doing their best to make the old custom games compatible with reforged. World editor will be the same with some improvements so you don’t have to worry about a lack of great custom games. There are however some issues that comes with better graphics and there’s also some major issues with the old editor that I hope they will address. I already listed some of these things in my thread: Thoughts about wc3 changes
I’d expect that the custom games that will take most punishment from reforged are the more ambitious ones that uses custom scripts, models, textures etc.