The future perfect that you’re thinking of is made with the verb will, not might, and with a past participle, not an adjective: All six lines will have been opened by 2020.
You can use the future perfect when you are mentally projecting yourself into the future and then saying that something has been completed at or before that future time (the past in the future). But the future perfect is not used very often. It is more common to predict from the viewpoint of the present.
All six lines will/might be open by 2020.
Your sentence with might would make sense in this context: One thousand years from now archaeologists are studying the remains of the metro system. They’re not sure when it was opened. Were all six lines open by (= before) 2020, or did all six lines open sometime after 2020? All of them might have been open by 2020.