Really, there just isn't enough information to answer the question - even if the space was properly (and legally) finished.
How big is it?
Do you have more than one bath?
Does it have an access that doesn't interfere with another room - especially another bedroom?
Does it have good heat and especially does it have good AC?
Of course the extra room doesn't HAVE to be a bedroom - it could be something else - office, playroom, hobby room. Extra space is good but some extra space is better than other extra space.