As long as it makes each game on a particular franchise its own unique experience and it doesn't feel forced to be there for the sake of it, formulas should always toyed around to keep things interesting, games like Kingdom Hearts, 2D and 3D Marios before NSMBW and Galaxy 2 respectibly, and Zelda are excellent examples of how you should go about making sequels.
A sequel that reinvents absolutely everything from the previous game has no bussiness being a sequel rather than being its own game, and sequels that change absolutely nothing from the previous installment but instead improve on everything have no bussiness being sequels instead of DLC.
I'm now filled with determination.







