Currently there is one workspace in Modeling, often this is not a problem - we are able to have multiple sets of data in one workspace and still logically separate them. Often though, things just get too messy: too many workplanes, too many random parts not yet part of a particular assy set, sets that are just completely unrelated making Save All At Top packages unwieldy, and auto-saves unnecessarily large.
Perhaps the solution is to allow multiple instances to be opened at once, but that may be tricky with multiple ModelManager instances. Perhaps the solution is tabbed environments in Modeling, with a way to move parts assys between tabs, and the ability to choose a tab to load to from MM.
This would also be great for Annotation, so you can be working on a drawing, and load another drawing for reference, or for helping someone with a question about another drawing so you don't have to save-load-look-reload-resume work.
Drafting already has the ability to open multiple instances, but my experience with it has been fluky and problematic.