Who is the authority that polices what aspects of a game MUST be kept for it to be a sequel?
Where does innovation become interference? I ask again: Who is the judge?
You use an audience to judge similarity. You have the intended audience of the series attempt to recognize degrees of similarities between games without first knowing of an intended relationship between any. The same way that trademark/likeness/copyright (when not applying to whole works) infringements are measured.
To just go and remark on the lack of a specific judge is to render the term "sequel" arbitrary and moot. Humans can identify patterns, if you refuse to acknowledge that, then you can make an argument for a hello kitty horse adventure game being labelled as a dungeons and dragons game. Who's to judge?
"interference" is a good/bad judgement call and I don't think it applies. There's a difference between innovating and taking only a setting. If a reasonable audience sample cannot gleam a relationship beyond setting without first being informed the relationship, then it would be more suitable for a spin-off classification.
I'm not saying Fallout 3 is only taking a setting. I'm still on the fence with it. It has action points, but I don't know if they're really similar to action points in more than name. It does have a limb targetting system, too. And probably other similarities, but I don't know how similar they are.