Feature request: share metamask private key with 3rd party

Description:
I am working on the mobile app which operates with chain and have to sign transaction by user. For PoC apps should support MetaMask wallet. Therefore I need a user’s private key.

The issue is originally MetaMask is a browser plugin and there is no direct way to get it from mobile app. However I just discovered MetaMask offers their own Android client and Android supports the variety of options to share data from one app to another one.

Purpose:
Allow MetaMask users from 3rd party app work with chains to sign their transactions.

Extension/Mobile/Both:
Mobile

Images/Attacments:
I still think about possible option to do this as each option opens its own vector of attacks. Let’s think more about it.