I think using the arranger window for more than just arranging patterns would be a good idea.
Eg Piano Roll should be able to be used in both pattern and arranger screens. I think using a PR used in arranger screen would work easier as you would just be able to jam alongside what has been put together already, without having to go back to pattern editor (if you wanted to)
Also i think audio tracks (and automation tracks if will happen) could be used in this way