Answer:
Nothing comes after Junior when Senior and Junior are used in naming members of one family. The use of Second (II), Third (III), Fourth (IV), etc., is a completely different system.
Joe, Sr., could be Joe I, Joe V, or Joe VIII or whatever number is appropriate. Junior and Senior are used only to distinguish between two living people. When Joe, Sr. dies, Joe, Jr. can become simply, Joe. Similarly if young Joe, Jr., dies, Joe, Sr. can become simply Joe.