Favorite "original titles" that never got a proper continuation are Bloodborne and Sekiro. There are many other greats that I just don't think they needed sequels (Team ICO games for example).
Favorite "sequels" that didn't get it: Too many to count but ZoE2 is at the top. ZoE2 was a huge improvement and it's a crime that the series ended with it. Well, Konami is in the gutter now.