Implement OpenID Connect
Having support for OpenID would enable pleroma to act as an identity-server, which could be used for a "sign-in-as" option. Last I'm aware of is that GNUsocial already has support for it.
There already are implementation of it OpenID in Erlang and in Elixir. The Elixir variant does seem to be built on the same webframework that pleroma uses, but it has been archived for some reason.
This has been referenced here as a possibility for a sign-in option, but I don't think it's a good Idea to close an issue when it hasn't been veritably resolved yet.