Using custom code is great, but it's difficult to tell how something is going to look when rendered (if anything at all). It would be great if you could upload a placeholder image for custom code to show visually what it will look like in the published website. This would be helpful for teams working together who might not be aware there's custom code in certain places that renders a UI.
I know that a difficulty with this would be responsiveness, so that would be a problem to solve. Maybe allow multiple placeholders for different viewport sizes? This would be helpful for me and my team as we embed Web Components throughout our website.
Ideally, I'd like to be able to execute custom code in the designer, but I know there would be a lot of security concerns with that, which I'm assuming is the main reason the code is not executed. I can think of other reasons, but I'm guessing that's the main one. If security is not an issue though, it would be nice to optionally execute custom code in the designer.