A character I definitely fell in love with (more) because of fandom was James Norrington from PotC. I liked him in the first movie but was very much focussed on the three main characters. But there was so much thoughtful meta about him, and also so many fics that used him in great ways that I really started to get invested in him, especially of course after the second movie came out. Sparrington was my first slash ship. XD (And yeah, I guess fandom also made me love that ship.)

Fandom also made me like Mystrade from Sherlock, which I hadn't even thought of while watching the show. XD (I mean, why would I, they share like 5 seconds of screentime. XD)

I also got invested in Xiyao because of fandom (might be similar to you and ChengQing) - I liked that ship on first watch but was so focussed on Wangxian and the Yunmeng siblings that it tended to be a bit forgotten. Fandom has done so much with that ship, but also with the characters indvidually, so much good meta about motivations and flaws etc. Generally, CQL/MDZS fandom has such good meta!

I think fandom never actually made me hate anyone. I got annoyed by some things, though, definitely. XD Like, in PotC, I was super annoyed with the Sparrabeth shippers after the second movie, because they were everywhere and they were loud and they hated so much on the Willabeth shippers it was unreal. (And I was in my little corner of Jack/Anamaria or Jack&Black Pearl plus Willabeth being there as well, and it was just so grating on my nerves.)
But I think that was the worst of it.