If there was one champion that ruined game balance the most in League of Legends, it has to be Ashe.
Ashe was considered a God back in the day when giving mobility to champions was very, very, very sparse. From the beginning of her inception, Ashe had always been a go to adc because the other adc's at the time did not have the amount of kting potential she did, and gap closers/blinks and dashes were very undervalued because most adc were imobile.
Due to the existence of Ashe, Riot created much more mobile top laners (see Irelia) , and in response, had to design more mobile ADC, and now we see the clusterfuck of mobility in League today.
Sadly, Ashe's Kit is good, incredibly good, but 3 years of designing the game according to that first response they made to Ashe has made Ashe's kit really weak when you compare it to the meta. So really, riot needs to change other champions, because they balanced Ashe a long time ago for her position in the game, but her position in the game is one that is countered by almost every champion released in the last 3 years, and that is something that cannot be fixed by changing her kit.
If they created Ashe differently, most of the above may not have happened.