Thanks for you interest. Such features are not in our roadmap, the goal of Map Journal is to guide the viewers through the story designed by the author without requiring them to perform some search or toggle on and off some layer manually. That's a key difference with the Basic Viewer that let the user more free. I'm not saying we will never add such options as in some case they can be really useful but from now the way to think about a Map Journal is that the viewer can only scroll through section or click on Main Stage actions and only those will change the Map content as the author wanted.
I'm sure you noticed that the builder allow you to configure the Map Content. We may add the Basemap gallery too but from now you can already add multiple basemap to your map from ArcGIS.com Viewer. Then you would configure your section to toggle on/off those layer.
Also one thing is that you can create a basic viewer for example and use it on your Main Stage.
If you are a developer, the code is on GitHub. Let us know what you end up with!