Abandoned games: An Aliens-themed RPG, and James Cameron's The Abyss

VG247 (which posted footage):

“The biggest challenge we had was how to keep the fear going even in conversations,” Avellone remembered. “You can make conversations stressful and frightening. How do you do it so that an alien could be attacking you at any moment? You can’t take shelter in a conversation with two talking heads while you try to figure out what to do.”

The game was a fair bit into development and featured conversations, combat, and even hacking. Much like Mass Effect, combat would be real-time with two AI characters supporting, and you could give them simple orders to get them to engage specific enemies or hunker down

And speaking of canceled games, files from Infocom's game based on The Abyss.