Experts and BHG readers answer.
How to put in attractive door separating livingroom from bedroom in a 60 inch open doorway
You could install pre-hung 2'6" double doors which would look nice and if it was eventually turned back into a den would work well. You could even do some obsure glass and/or some panes in the doors to give it a more attractive look. We installed these in a master suite addition that came right off the family room and it looks great.