the series doesn't have to stay completely the same like COD but it doesn't have to scrap almost all of the tried and true mechanics of the game.
They don't. You still play the game (largely) from 1st person. You still have a reliance on skills, and you increase your skills by what you use. There's still the 'do what you want, when you want' gameplay focus.
What tried and true
mechanics have they scraqed? Well, you could argue the randomly-make-your-action-ineffective (aka, dice roll) mechanics, but many people say that change was a good thing because it doesn't fit with an action-based first-person perspective, which is something that's also remained through the series. The mechanics of the game haven't changed because they removed some skills or weapon types. There's still more than a dozen skills spread over the three archetypes (warrior, mage, thief).