Microsoft provides the xrm.panel namespace as part of the Client Api since the 2016 December update as a preview feature. This allows for embedding a HTML client into the Dynamics MDA form on the right hand side and allows the user to fold in/out this section. This is perfect for embedding other customer applications or PowerApps into the Dynamics 365 CE experience. Customers are impressed by the possibilities here and this is easy to implement using TypeScript/Javascript.

At time of writing (2021) this is still a preview feature (hence: unsupported) with limited functionality to set a URL once (cannot be updated) and no methods to fold/expand the panel, unload the panel or interact with the content in the side panel.

From my opinion the ability to implement this Panel option will further enhance the user experience and user adoption by removing the need to open the other application(s) in separate windows.