Love the topic, VERY debatable.
Personally, I think Frye should already be in there. I think BJ and Randy are both a clinch due to each holding a belt in two different weights. That is the type of accomplishment that reaks of HOFer imho.
Anderson and GSP, no comment. If they're both not clinches, than someone needs to define what a HOF is to me exactly.
Rashad/Machida? Too early right now, but they're both on their way if they go another 3-4 strong years. I think Forrest deserves no mention in the same breath as the HOF, but I suppose his fight with Bonnar, and the impact it had on the UFC will make it hard to deny him at some point.