My favorite in Space Marine was when executing a Nob, you'd wait for it to scream then jam your chainsword into its mouth. Brutal. I'd love to see melee executions like that. Visceral and as gory as they should be - VERY. I'd also love to see some choke-lifts. Place hand on Eldar throat, lift said Eldar into the air, then empty a couple shots into it's head. Making said head disappear in a fine, red mist. Suitably horrific.