It would be great to have the new expression editor in the Interface tools such as Action and Error Message to have the modern expression editor, not only for highlighting and autocomplete but also the "preview result for the first row" (which is the only row when you are writing an expression for an Interface tool.
I think that the addition of this feature is especially necessary because of the "Update Raw XML with Formula" feature, which requires you to clearly be able to see the output of your formula, which in turn usually requires you to first test the XML in a separate workflow with a Text Input tool.