Robocop Rouge city does it perfectly. There is the main game which isn't long, probably 10 hours but then there is a load of side content that you can choose to do that feels as if it is main mission stuff and extends the gameplay to about 20 hours. Same goes for The Witcher 3, all the side content is of such high quality that it feels like main quest stuff but it's also completely ignorable if you want a faster playthrough. Stellar Blade again, another perfect example as is From Soft games.