William Wallace definitely makes for a good Servant idea, but why is he Berserker? For what I know of his legend, he doesn't strike me as the "mad warrior" type (patriotic and battle-hardy to be the sure, but hardly "mad"). Also why does his Noble Phantasm serve to conceal his identity (especially since it's probably the most famous symbol of his identity), and what do you the mean by "the blade has been altered over time"? Second NP is definitely fitting, though a tad generic.