I believe that Penumbra was an amazing series, although Amnesia has made second on my favorite game list (Zelda: Majora's Mask is my personal favorite tied with Ocarina of Time
). I believe that they should create a new game with a different story line - though the game should still have the same eerie atmosphere, darkness, and insane scare methods Amnesia has. Perhaps Amnesia can turn into a series of games that will forever be known as the scariest games created!
Personally, I think Frictional Games should create stories that link together starting from early times (Such as Dark Descent) to present day. However, present day may not be such a good idea since there are a lot more options for light sources (cell phones, flashlights, etc) and could ruin the game. Although, I think that one certain castle/dungeon/mansion or whatever, should be linked and used throughout the time line. Give your personal ideas