A sequel would normally improve the gameplay and scope of a game... but obviously this would be case by case dependant.
As for Xenoblade X, as dunkey would say, "Who gives a shit, it's an RPG, boys. Story doesn't matter. It's ALL about the action baby."
@d21lewis: No worries, everyone can have fun with MK8 now, I'm just more been there done that.
I don't know, the completionist in me wouldn't want to buy a sequel without playing the original. Unless it's a sports game. But, not everyone is like me.