Sequel First: On home computers, Deja Vu, Uninvited, and Shadowgate were released in that order. When the company decided to port to the NES, they decided to just go with the most successful one (Shadowgate) because the plots aren't really tied to each other and that offered the highest chance of getting a return on their investment. When Shadowgate proved popular enough, they then went back and ported the other games.
Vaporware: Shadowgate Rising for the N64. It actually got rather far into development before being canned, and had a tie-in comic published on Kemco's website.