Story Idea

So, I had an idea for a new story today. It takes place in a fictional world that's pretty much the same as Earth. Except, there are some supernatural elements in existence (e.g. some mythical creatures, watered down magic, etc.). The setting is on an island that's isolated from the mainland. It has a temperate forest climate, like Scotland or Ireland. The civilization is based around Pictish and Gaelic culture. People wear kilts, play pipes and drums, drink a lot of homemade alcohol, all the stereotypes. Naturally, the civilization would be apart from both, but would share a lot of elements. The basic plot-line is a tried and true recipe, but it's one that I like. Basically, a young man (when the story starts he's like 12 or 13) is afflicted with one debilitating disease or another, and visits a malicious medicine man type figure who cures him in exchange for some valuable thing (not sure what that thing might be yet, maybe a piece of his soul or his firstborn or something like that). Now cured, the young man rises to greatness over the course of a few years. Now, a group of invaders comes over from the mainland and the young man takes up the fight for his people. At a pivotal point in the war, the medicine man comes for his due. The young man denies him for one reason or another. Then, the medicine man curses him, and goes on to betray him to the invaders and they (the invaders) win a major battle and establish a fortified settlement or other major foothold on the island. With more and more troops pouring in from the mainland, the natives' job of holding on to their land becomes that much more difficult, and everyone knows it's the main character's fault. He is banished to an unsettled part of the island, and that's the end of the first story arc. Thoughts? Opinions? My train of thought is an overall plot synonymous to the conquest of Britain by the Romans. Except this time the natives win!