1. Macho Man- If he made a surprise return at the rumble in the past 5-6 years (shyt probably in the last 10), the pop would have been ridiculous. Probably would of been one of the most memorable moments in wrestling history, especially in recent history. Him hitting the top rope elbow drop on Punk because he has been using it, and tossing him out... 
2. Bret Hart and Owen Hart- Injuries and beef made it impossible...but a Bret Hart in reasonable wrestling form would have been quite the show. We all know what happened to Owen..but with the rumors of him retiring early had he lived...chances are he would have been coaxed into a brief return at some point (even if it was a one time appearance).
3. Jake the Snake and Scott Hall- DDP yoga has been inching them closer to making it a possibility, but it is still close to 0% chance of happening. Bunched them together due to DDP..but either one would have been great to see, had they not gotten so
..
might as well throw Luger in the mix here as well. 
Others: Pillman, Rude, Meng, Bam Bam, Boss Man, Arn Anderson, British Bulldog, Yokozuna
.

2. Bret Hart and Owen Hart- Injuries and beef made it impossible...but a Bret Hart in reasonable wrestling form would have been quite the show. We all know what happened to Owen..but with the rumors of him retiring early had he lived...chances are he would have been coaxed into a brief return at some point (even if it was a one time appearance).
3. Jake the Snake and Scott Hall- DDP yoga has been inching them closer to making it a possibility, but it is still close to 0% chance of happening. Bunched them together due to DDP..but either one would have been great to see, had they not gotten so
..
might as well throw Luger in the mix here as well. 
Others: Pillman, Rude, Meng, Bam Bam, Boss Man, Arn Anderson, British Bulldog, Yokozuna
.
...that pop when he eliminates Brodus and Santino
....dem boos he gonna get after Punk eliminates him 