Add AWS EC2 instance to existing Availability Group
I assume it would be like this hybrid it: disaster recovery scenario but with AWS instead of Azure. There is an interesting point in the article about having your Active Directory available in the cloud. 

I think you'd have to give some thought to how quorum would work. For example, what would happen if you did a manual failover to the cloud instance, then later your on-premises instances came back online? 

HTH