But 20% is more than it seems (it can be 1 totally unique gameplay element, e.g. with Painkiller (2004) tarot cards and demonform). Besides ones a few games copy each other succesfully and keep changing ~20% (/ +- 20%), you will get totally new games fairly quickly considering how fast new games come out. Than ofcourse there are a lesser amount of games that innovate alot which either fail (C&C 4, besides the fact whether one find it bad or good) or become a success (Dune 2, Deus Ex 1, Mount&Blade 1?).