I bought the top of the line 12.9 today. What can I say, I am planning ahead for a future of Photoshop and Xcode (one can dream).
It is fantastic to hold and to use, especially with no case on it. I only purchased the Smart Folio to go with it for now, partly because it is pretty light but also because it is so easy to remove. I am mostly using it bare.
I expect them to support USB drives in the future. They cannot not do it and still compete in the pro space. And technically it is such low hanging fruit now. They are getting reamed about it in reviews. I don’t think they will have a choice.
Robust multi-monitor support is non-trivial to do really well in the short term. iOS just was not designed as a multiscreen OS, and iOS apps make certain presumptions about how they are displayed, along with how and when the displays change (rotation, etc). There will need to be more OS and API changes in future releases, and apps will need to conform in order to behave well. I hope that Apple pursues those changes and that app makers take full advantage.