Take everything you already make without guns and add guns to it.
Rare games, Shadow the Hedgehog, Jak and Daxter
Games that went through a drastic mood change
Jak and Daxter, a few Sonic Games, Conker (a rated E and a rated M game)
Games that become a powerful franchise, but runs on a story that stops dead
What will ever happen to Metroid without Metroids? Will there be a clone of Zebes?
Game who uses their characters like memes
I love rabbids, but they've been multiplying like rabbids.
Story based games that are losing their story touch
Final Fantasy are lacking recently
and 100% what ruins a game series is when the game is no longer about the game, but about the game formula!