What I mean as "Commonwealth" is an intergovernmental organization which can trace its origin from European colonial empires in 19th century, thus most (but not all) of its members are former colonies of the European nations, e.g. British Commonwealth and Francophonie...
Your challenge is to make present-day Spain, Portugal, Netherlands, Germany, and Italy have their own Commonwealth organization...
(the PODs are up to you, but preferably no earlier than 1492...)