Expanding The Keep On The Borderlands

I'm playing again with my friends this week, in our Keep On The Borderlands game I run under BX D&D (they're all newbs, so y'know, keep it simple stupid). Since I really look foreword to turn it into a regularly scheduled campaign, I decided to throw in some background elements in the game. Until now they have spent almost no time at the Keep, and only explored the Kobold Caves. So I brainstormed a little, and this is what I came out with.

I will scale the campaign world to the size of the Middle-Earth. I want one region controlled by the Elves, one by the Dwarves, one by the Halflings, two or three by the Humans, and one by the Archvillain and his minions. (Plus a couple of countries that doesn't show on the main campaign map). The Black Barony will be part of one of the Human kingdoms, so that all of this work can be recycled for my Tower Of Zordaz campaign.

The idea is that some hundreds of years ago, an evil army of chaos lead by the Archvillain (or his master, or a relative of him, or, well, you got it) emerged from the wilderlands behind the borders and attacked the Human Kingdom, carrying death and destruction. The war lasted for almost two years, and eventually the army of chaos was defeated and forced to withdraw. To keep the borders protected in case of a new assault, the King of Men built several strongholds, erect to stand between the forces of chaos and the free people of the continent. Unfortunately, the Humans are quick to forget given their short life span. A great civil war fragmented their reign, and the power of mankind were considerably lowered. What is to happen, now that a new evil is rising from the dark?

I do not exclude to throw the Rod Of Seven Parts in somehow. It's such a powerful (and iconic) Artifact that I could easily make it the Archvillain ultimate goal. (Or at least, something the adventurers are willing to obtain to lower down his plans). I know the whole stuff is nothing really original, but I think it'll work just fine. Also, what I really want is to let my players do basically what they want, while the Archvillain pursues his own goals, so I hope this really easy and linear plot won't screw me up too soon.