The basic use of the 80-20 rule (Pareto’s principle) in video games is to copy the 80% of a successful or promising game in order to mitigate some of the risks involved in investing time, effort and money in developing it.
Then you have 20% of freedom, where you can put your own secret sauce to the cake.
The risks of this approach come when you do not understand WHY a certain competitor works out there. It may be a successful marketing case. Maybe a special UX is what makes it great. A great level or narrative design may be making the difference. Hard to tell if you do not know basically three things:
The first thing is to deconstruct competitors,
The second thing is to run playtests with competitors games,
The third thing is to study the audience of competitors in a meaningful way.