With PowerApps all security is managed at the data source so that App Makers do not create security overrides within the app. In some cases, you can also share custom connectors to expand access to data from your apps.
The "impersonate user" capability in SharePoint Designer is similar to what is possible today using Microsoft Flow. In Flow, you can assign accounts on an the level of the "Action"s performed.
We will consider feasibility for something similar to what is possible in Flow using PowerApps.