Well, I have two series that started incredible with their ideas but eventually they... they took a train, it set on fire, they fell down a cliff... and it exploded and then an alien shoot a laser at them.
Supernatural, first 5 seasons, amazing premise, eventually each season had incredible and interesting ideas in concept but the execution was poor.
Same thing with Wonder Egg Priority, it was so well done at the beginning, the art as well was -cheff's kiss- but the latest episodes and it's finale... ugh.
Well, Riverdale started ok-eish and went sideways, did a flip mid air and God knows what would have happened if it didn't got cancelled.
Yuri on Ice, as other people have mentioned, I also considered it a poorly executed idea... it was just grabbing your most boring and vanilla takes from both sport and BL anime and done, nothing thrilling or that could be called so amazingly executed.
There were also people who mentioned Vampire Knight and I'll be honest, I totally agree with that take and this is my favorite video for anyone that hasn't seen that show and doesn't want to risk their lives and braincells in the process
And I'm going to be honest here, anything from CW right now is pure good ideas and horrible execution, Gotham Knights feels like Descendants from Disney but with a Batman mask made with a 3D printer... and Winchesters... Feels like Riverdale and Vampire Diaries. I don't know how they are going to execute those two but again, it's CW and after Supergirl, The Flash, Riverdale, Charmed, Supernatural and several more, I have no hopes