According to Tales of the Jedi, one of the main reasons Count Dooku fell to the dark side in Star Wars was due to the Jedi offering Mace Windu a seat on the Jedi High Council before him. Between 50 BBY and 42 BBY, both Dooku and Windu were assigned to a mission involving Master Katris murder, a former member of the Jedi Council. Because of Dookus actions, the real truth about Katris death became apparent, but at the cost of Senator Lariks life. Afterward, the Jedi Council offered Windu Katris seat, despite Dooku having uncovered the truth.

https://screenrant.com/star-wars-what-if-count-dooku-jedi-council-not-mace-windu/