I just wonder if the format is considered done with no further improvement needed? If not are there roadmap? And are the different version of MKV standards?

There have been changes and additions and I guess new ones will turn up in time. Just recently it got new features needed for multi-view-videos, as the old possibilities turned out to be insufficient. (v3) I'm not aware of any roadmap, though. (Mosu might)

I am just wondering if they would ever consider Streaming as something they could think about.