I'm glad that you brought this point up! I've been waiting for awhile to see wrestlers in WWE use foreign objects again..Obviously HHH will use his sledgehammer in one of his rare streetfights and Santino has the Cobra, as asinine as the Cobra is, but besides that there's no one else today in WWE who uses foreign objects anymore. As to where the brass knuckles have vanished to, I suggest you call William Regal because I believe(correct me if I'm wrong) that Regal's the last WWE wrestler to use brass knuckles..Back in the 1980s when I first started watching wrestling, most heels would use foreign objects, ie Rick Martel with the Arrogance cologne, Jimmy Hart had the megaphone, Mr. Fuji had the ceremonial salt, Sensational Sherri would use her high heels as weapons, Paul E Dangerously had the 80s cellphone, Jim Cornette had the fucking tennis racket, Abdullah the Butcher obviously had his forks among other weapons, etc,
My point in stating this is that with the right heel, using a foreign object could bring back that currently missing element of heel psychology which usually got the crowd riled up if done right. The way Abdullah would use his fork to cut up his opponent was that he'd use the fork except that his body would prevent the referee from seeing it. Classic, simple and highly effective tactic.
Here's a couple of heels who I think could use a foreign object
Randy Orton-He could use brass knuckles. I could definitely see Orton using foreign objects in order to keep his title by "any means necessary" since it fits his desperate character.
Damien Sandow-Even though he's a tweener jobber at the moment, if he was turned heel again and put at least into the upper midcard, I think that he could use the roll of quarters..I haven't seen that used since the 90s.