Current status:
We have a 2 node WSFC with 5 SQL instances running on it.
Plan:
The plan is to implement AlwaysON,with a DR server in other data center.
So,can we have databases from all the 5 FCI in different AG's(one AG for each instance) on the DR server?
if all nodes are part of the same cluster, then yes. As AG requires all the nodes to be on the same cluster, if that requirement is meet, then you can have all 5 AG failing over to the same DR node.