I really liked Dortmund in the Klopp era and wanted them to do well, but since their player exodus, it's become increasingly hard to root for them. They never learn their lesson and continue to sell their best players to their rivals.
The sad reality is that if Weigl develops into a World Class player, he won't stay at Dortmund. I won't call them a feeder club, but if Bayern comes in for a hypothetically world class Weigl in 2018, Dortmund will not be able to refuse unless there are some serious changes at the club.
No one remembers the team that comes second as they are confined to the washed up history books. Dortmund's players will continue to get fed up of the club finishing second place all the time and demand to leave. High wages aren't possible at Dortmund either to offset the discontent.
He is a phenomenal talent, and when(not if) he decides to leave, Manchester United must be in the running for him.