For example, I am wanting to make token copies of a legendary creature such as Progenitus using something like Progenitor Mimic. I know this is illegal by itself, but does there exist a card that would allow this?
The card you are looking for is Mirror Gallery, which is an artifact with the text
The "legend rule" doesn't apply.
In addition, Lazav, Dimir Mastermind and Sakashima the Impostor become copies of other creatures, but they keep their own names, so they can copy other legendary creatures without dying due to the legend rule.