I'd suggest:

1. Looking at different industry verticals where a truly synchronized viewing of documents is of use
2. Then arrange those in a descending order using a mix of variables such as size of market, perception of pain felt by client, etc.
3. For the verticals that come out among the top 3-4, it might make sense to do another round of customer development interviews to validate your hypotheses around utility to customer

This could potentially point you to a small number of compelling use cases.

I'd be happy to look at (a) what use cases you currently have and (b) brainstorm/be a devil's advocate as you go through the process above.