Using the term "unknown villain" mischaracterizes the situation in such a way as to inspire mental laziness.

I much prefer the term "barely known villain" which is both more accurate and begs the question, "What IS known about villain?"

***
I don't see any context in which a "default play" is relevant to playing good poker. The "default" play is found upon reflection of how you have handled many situations against a specific player or player pool. It is not a predictive piece of information, nor is it transferable between players or across sites.