Working with Individual Contact Suffixes
I consider things like RN and MSW to be qualifications that should be dealt with in some other way rather than adding them to the suffix field. 

A contact is always Charles Montague III but he is only a RN whilst he maintains his regisitration as an RN which will involve a certain number of hours of professional development and practical work each year.

If you need to be able to search for people with a particular qualification then a custom multiselect field to deal with them would be the best option I suspect.  The final option could be Other, and you would have an associated free text field so they can specify exactly what you have left off your extensive list. 

Alternatively, if you just want to be able to tack these on after the formal name, then a free form text field may be easier.