Here is an interesting topic, I'm not asking which game series you want to die out because of whatever reason.
But instead, which games do you not want to see sequels for, because a sequel will most likely ruin the original game in some form or another?
A notable example for me is Dawn of the New World. The original game was a game I really enjoyed but after playing through the sequel, it really ruined the whole original story. By trying to add another layer of summon spirits and plot twists (which worked really well in the original), not only were they weren't able to replicate how it worked in the original story, but just ruined the whole point of the original game. Plus the second game wasn't good all in all
So for my pick, I would go with The World Ends With You. There was a mobile spinoff or something, but don't really know anything aside from that so it doesn't count. The original game had a great atmosphere, story, and unique gameplay and I feel that a sequel won't be able to match up to that standard and would just be inferior to the original.