Lastly I subscribe to the school of thought that says there is such a thing as too many sequels. This is true for movies & games, even TV series. I feel the optimal number is 3, maybe 4 if ur a good studio. After that, move on, try something new otherwise you run the risk of over saturation and the death of a decent franchise.
COD, NFS, Star Wars - The Phantom Menace
Mass Effect has the winning recipe down by not going further than 3. There must be absolution. Open book endings turn awesome games into Twilight sagas.