the big takeaway here is that if the NFL wants to elect a special teamer, it needs to nominate Rick Upchurch, not Ray Guy.
What do you think about Billy "White Shoes" Johnson? Now that Guy has made it, Billy is the sole member of the NFL's 75th Anniversary team who is not in the Hall of Fame. I also thought of Terry Metcalf when you mentioned Rick Upchurch, but browsing his numbers, it seems he was far greater in my mind as a little kid than his production indicates. He sure was exciting and slippery though.
White Shoes is another excellent choice. I think Upchurch, Johnson, and Mel Gray are the three greatest career returners of all time (setting aside Devin Hester for now- we'll re-evaluate when he retires, but he could easily join that group). I think if the Hall was dead-set on electing a special teamer, any of the three would be a worthy standard-bearer.
When splitting hairs between them, Gray and Upchurch both get an advantage for their hardware- both made the pro bowl 4 times and were 1st-team AP All Pros 3 times, while Johnson only made 3 and 1, respectively. Gray was the most involved on kickoff returns, but he was BY FAR the least involved on offense (263 yards for his entire career). Upchurch and Johnson finished with eerily similar offensive numbers (~4500 yards and 27 TDs), but Upchurch did it over a shorter span and had the best offensive season of the trio in 1979 (950 yards and 7 scores, which actually made him the #14 fantasy receiver that year).
So, Upchurch was the best on offense and the best pure punt returner, but didn't return kickoffs after his first few seasons. White Shoes handled kickoffs off and on, and was also quite involved on offense, but had the fewest awards and wasn't quite as good on punts. Gray was the worst of the trio on punts and did absolutely nothing on offense, but was the best and most prolific kickoff returner of the group, and matched Upchurch in awards.
Really, if the Hall of Famer was dead set on electing a special teamer, I think any one of those trio would be a fitting standard-bearer. I think Upchurch has the best resume of the group, but all three are close and are also head and shoulders above all the other returners, at least until Hester finishes writing his resume.
Edit: I dug up
this old post from the PFR blog which has Gray, Upchurch, and Johnson as 3 of the top 4 returners of all time, with the fourth being Brian Mitchell. Mitchell had a really, really long career (especially for a returner), but didn't seem as dominant as the other 3, with a lot less bold text on his PFR page and only 1 pro bowl / 1 first team AP All Pro to his name. With Upchurch rating as the better returner, having more awards, *AND* being more valuable on offense than White Shoes, that seems like a clear edge. From there, it's a question of whether you'd prefer the extra value Gray added as a returner, or you felt that Upchurch's offensive contributions outweighed it.