If a game is going to be a sequel then it should have everything the previous version had and extra. Hence, why they are sequels. Good sequels are NWN1 to NWN2, BG1 to BG2, FO1 to FO2, and so on. Deus Ex to Deus Ex: Invisible War is a bad sequel pairing as well as Fallout 2 to Fallout 3. When a game is a sequel to a previous game it should have the same core design, but expanded and more well defined, not completely changed or removed.
If you want to completely change the core design or remove elements from it, then don't make a sequal. Make a new game instead.