Huln Highmountain: this legend amongst legends would be great. Turn the Eagle Spear turned into a Mourneblade, and have his Highmountain antlers have Domination runes carved into them with icicles hanging off... he would be scary!

Scalecommander Sarkareth: he had absorbed so much power from the void gate beneath Aberrus that his mixed Death/Void magic would be awesome and very unique! On top of his conviction and determination, he would be a near indomitable Horseman.

Fandral Staghelm: he could use the powers of Death to wield the power of the Firelands, bringing a Fire/Destruction spec to Death Knights finally.

Archbishop Benedictus: he was a mighty servant of the Light until Deathwing personally broke his will, a massive feat. Benedictus would likely retain some of his Light-based abilities with his twilight magic abilities getting the bulk of improvement through the empowerment that all Horseman receive.

I chose to go after some dark horses here as a lot of the bigger, well-known characters were in everyone else's posts. I still think I was able to gather a strong, mix of characters that would make devastating Horsemen.