Well, that's pretty much what the iframe object can already do, but securely by default. Is there any particular reason that doesn't do what you need?
One of the reasons I've been promoting the HTMLElement plugin is because it allows users to click on various HTML elements and pass those clicks to Construct functions. THAT alone is a HUGE step above a simple Iframe. HTML is a coding language in it's own right. It took me 2 days to get the code in Construct to work and display 2 layouts, make Ajax requests, parse Json results, etc. It took me over a week just to get the HTML for ONE GUI element working well enough that I was satisfied and, as you saw in the link above, I had to create that on codepen.io. Customization requires extensive knowledge of CSS and JS and even CSS is becoming a massive nightmare trying to keep up with all the browser specifics. If I want to display HTML, sure I'll use an Iframe. If I want to create a GUI, the Iframe is damn near useless.
I have mentioned that I still program in an ancient language called Pascal. Delphi is the IDE I use to develop in. Delphi is called a RAD (Rapid Application Development) platform because you can quickly build an app with simple controls. I have literally 1,000 elements that I can drop on a form, everything from visible buttons and scroll boxes to encryption methods and pie charts. The publisher of Delphi only created a handful of these. The vast majority were created by the community. The reason there are so many of these plugin modules is because they too are created in the Delphi environment. Once you learn how to code in Delphi, you can create these plugins.
I know how to code in Construct but I can't write a plugin. I took a brief look at how to write plugins for Construct and threw my hands up in disgust. Even a basic 'hello world' plugin was going to take a huge amount of effort just to learn JS beyond the basics.
Delphi and Construct have one thing in common that attracts me to both. Rapid development. I didn't have to learn the intricacies of any language to begin building with Construct. I only had to rethink some syntax.
My suggestion is something I understand you've already considered. Make creating plugins easy. Make it so that if I have a knowledge of how to use Construct that I can create my own. You give me that power without having to spend months learning JS and OOP and callbacks and not only will you see me creating GUI elements, you'll likely see the plugin collection expand massively.
YOU do not have to reinvent the wheel here Ashley You only need to give your Construct users the ability to create their own wheels with a limited amount of facedesking and I'm fairly certain that the work that's being suggested here will be handled by your community.
Here's the proof of the concept.
They currently have over 600 'components' for Delphi in their open source library. All of them created by the Delphi community.