Generally the rules for eligibility to a Hall of Fame is that the character/actor must have left the show for a certain period of time (say 2 years+) so you couldn't add any current characters or say, Lou who didn't leave that long ago. Sporadic short term re-appearances wouldn't prevent eligibility though, it's just when they are still a permanent cast member.
As for how you assess if someone qualifies, I guess it's about their contribution to the show - how memorable was the character, how many big storylines were they in, did you have a strong emotional bond to them.
I would start the Hall of Fame off with Harold and Madge Bishop, entering together.
Who would you add, bearing in mind it shouldn't be over-crowded and only the very best should get a place!
