I honestly loved this show so much, it is my most favourite show of all time. I loved delena, it was amazing to watch them, their chemistry was amazing. Not to mention the villains in the early seasons were very intriguing to watch, they had so much depth and had an amazing journey. Katherine will be my all time favourite character in the series and villain in the entire series, even if she was very self-centered she had moments where she was vulnerable and had a sad backstory which added so much depth to her character arc.
SPOILERS AHEAD
They did ruin her character in s5, when she was on the verge of death, she should have just died because she had such a happy moment with stefan, but julie had to ruin it and make it that katherine would hijack elena's body.
Or since stefan actually started to like katherine again in s5, he should've found a way to stop the aging process after taking the cure to immortality. Then doing so him and damon could've done the same thing where katherine and stefan could've lived a long human life with each other, where they got married and had kids. The same thing with damon and elena in this universe of my theories/thoughts.
I also feel like it should've only been 5 seasons with Silas as the final villain. Katherine getting a redemption arc where she saves everyone or something. With my other theory about steferine combined with this it would've brought her character in a full circle giving her character the resolution a lot of people desired and one she deserved. Like them bringing her back in s8 just to kill her off again was so stupid, i was so pissed, either they should've kept in 22 episodes so that katherine would've had more screen time or did that s5 as the final season approach and a long human life with stefan.