An angry grandpa protects a teenage girl from mushroom people.

This thread could be even better if the poster bellow had to guess the game above and if people stay away from critical spoilers.

A really nerdy scientist goes back in time and makes the cold war even worse.

You play as a cat thing who befriends a robot and they travel across the galaxy tryna stop some guy from destroying the universe.

Some little elf boy in a dress is in a perpetual state of having to repeat daily chores and interactions, only for the events to rewind. This leads the the elf boy in the dress having to fulfill said tasks over again hoping to gain different results. If the boy does not make good use of his time, his efforts are all but for naught, and he must repeat the cycle over and over again, in a never ending perpetual cycle of repetition and monotony. Occasionally something different will happen due to your actions, but it doesn't matter, because you are bound to reverse time again, thus making your actions redundant. Best game ever.

Blue haired Gary Stu saves a princess, frees a nation, and kills a goddess just because he fights for his friends.

That actually doesn't sound too bad...but it's the best I can do.

You have an accident and get turned into a monster where you fight other monsters to inevitably die by the end. Then you come back to fight more monsters and get revenge and the moneys.

No story, no dialogue, no enemies, you spend hours riding a horse and nothing happens. Only a handful of bosses in the entire game, and the entire game hinges on your unbreakable vice grip.

Another pointless war with people that have no logic

(Fire Emblem Radiant Dawn)

A guy that can't speak runs through the same 5 castles 80 times dying over and over again so he can kill a slug under his home.
A guy shoots walking mushrooms to save the world, and then decides to not save the world at the end.
You shoot enemies to make flashing colors appear on the screen everywhere and give yourself a seizure, while an annoying robot makes whitty one liners.

You play as a globe trotting mass murderer, and make whitty one liners to your 60-something friend. 
A guy named Big Boss tries to kill the boss. (The actual names of two characters). Also, everyone is bisexual.