I would LOVE it if in repls that serve a website, we could navigate to different routes in the embedded preview pane. Maybe this is already possible? For me, it seems stuck on the "/" route (Flask is all I've tried).
In my browser, I can edit the element
<iframe sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals" src="https://<repl.co url>/<custom route>" class="jsx-1342728394"></iframe>
to have a custom route, and it instantly changes, of course. A better interface would be to make the route editable (imho). The best interface would be if only the part of the route AFTER the "/" were editable in this pane. The URL in that pane is technically displayed in an <input> element, so a 2-way binding may not be too much of a stretch.