Well the first that comes to mind is one that simply gives him physical strength, sort of like what Shai used in The Emporer's Soul. Or, it could could make him much smarter and knowledgeable. However, those are both a little basic, and I think if he had worked on just one essence mark during his training, it would be more personal. I’ll have to think on that. A basic one (or two) might be the most useful, in the long run.