Yeah, it takes a LOT to ruin something I love...in fact, I can't point to things in any series I like that make me go "RUINED FOREVER." Mostly because I tend to judge things based on a per game/episode/book/whatever basis...but, I have some near-misses:
Ultima. Nearly screwed by
Ultima IX: Ascension. This game is the single reason I will
never have any love for Electronic Arts whatsoever. Not only did they halt
The Lost Vale, the expansion for Ultima VIII which had the potential to make it go from okay to epic, but this game was an absolute, utter travesty.
A list for every single nitpick the Ultima fanbase has been able to find in Ultima IX. There's 506 of them in total...that's more than
every other fucking game in the series put together. It had the potential, much like its direct predecessor, to be an absolutely
breathtakingly epic game...one of the first 3D RPGs with a large, open world. So much potential...fucked by EA. For that
one blunder, EA can burn in gaming hell for the rest of eternity.
Thief. Nearly screwed by
Thief: Deadly Shadows. Now, Thief III was a fair game in and of itself...but, it suffered a very similar problem to Ascension: excessive executive meddling. Ion Storm, if I remember correctly, was originally going to make their own, homebrew engine specifically
for this game. But, due to executive bullshit, they had to kludge a stealth system into the Unreal engine. Even then, it could've been an excellent game, worthy of being a descendant of the most excellent Thief II, had it not been for the seriously buggy release, initial lack of modding tools, and, worst of all...the Keeper Enforcers. These guys had the potential to be thoroughly
terrifying enemies, but, as it would turn out...they were nothing more than retarded killing machines who would spend most of their time killing innocent civilians.
I'm sure there are more, but my brain's gone to Uselessville on me, unfortunately...