OK, since Burger opened the can of worms that I was trying to side-step [s]so that I could throw our glorious leader into the fire[/s], I will give my opinion on what you could do with Samoth.

Go back to his original incarnation and basically make him a paladin (ala dragon age) who resists magical abilities and fights back with the physical. His core being magic cancellation, and then his abilities all have to do with surviving magic, growing stronger from it, or just naturally being a beast physically. There, it's all thematic and focused, and better yet still stems from one of your ideas.

Would he still be incredibly strong? Yes, but now he has a specific role that can be worked around to still stand a chance against, and he'd be great for the "Worf Effect," basically if it was ever necessary for the plot, he could lose to prove just how scarily strong some other antagonist is.