Frank Shamrock is the most obvious. Ken Shamrock deservedly got in back in 2003, before he became another one of Dana's enemies.
Since they added Rashad Evans, they should definitely induct Machida, who beat him and defended his LHW belt one time (as opposed to 0 title defenses for Rashad). He also had a nice run (including a title fight) when he first moved to MW, unlike Rashad who was a MW disaster. The same goes for Forrest (inducted way back in 2013)--he never had a successful LHW title defense and didn't have as many signature wins as Lyoto.
Wait, even Anderson Silva and GSP aren't in there? Do they have to be retired to be inducted? I assume not since Faber is still fighting. If Bisping and Forrest got in, then many others deserve to--even Tim Sylvia, who defended his belt more times than Bisping. Bisping's one title defense was against a 46-year old who was 3-6 in his previous 9 fights... Wow, the HoF is even a bigger joke than I thought.