Hmmm, a lot of games could fall on this category. Most recently, I would say DA2.
DAO was such an amazing game, combining the elements from Baldur's Gate and KOTOR, two of the best Bioware series ever made. The battle system was complex but accesible if you gave it enough time, and it had an epic story behind it.
Sadly, DA2 is just a shell of it's former being. Completely unthoughtful combat system, annihilating any kind of strategy or actually deep thinking, a story that literally pissed on the greatness of DAO, amongst many other things.
KOTOR II could fall into this category as well, but since the Restoration project has been released, I have no more qualms about it anymore.
The Third Birthday is also a game that could have had great potential, given the amazing background of the PE series, but it was a complete letdown, butchering the story and having no relation whatsoever to the PE games. It was a "sad" comeback after 11 years of waiting.
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