I personally hated my beautiful lockscreen wallpapers replaced with pixelated music album art back in the kitkat days and certainly don't miss it today
I can, however, get back the water effect lockscreen like the Galaxy SIII had with a third party option like X locker though. But I am happy with the dynamic lock screen option in One UI where it shows a different animal or nature landscape with each screen wake. Satisfies that classic Nature UX itch.
I do miss TouchWiz though. I can't get back my droplet touch sound, or the full skeuomorphic UI just with themes, old apps and widgets. It will never be complete. But I do get more options for look than I would get with stock android. But I want my notification ticker text back, my green battery icon and wifi bars back, and my button shapes in pop-ups over coloured text on a rounded corner white box...Oh, and I want my phone dialer to have glossy buttons again instead of borderless numbers on white. There are third party dialer apps that offer it, but all of them instantly end the call soon as you try to dial. Only the flat UI crap apps work