Given your hypothetical (two more 1,000 yard seasons), Derrick Mason = Charlie Joiner. Receiver who managed to remain good for a long, long, long time. Unfortunately, the Hall already screwed up and elected Joiner (far and away the most undeserving WR in the hall, IMO, miles ahead of Lynn Swann). The Hall wouldn't compound their mistake my electing Mason, too. I mean, for crying out loud, the guy has only made one pro bowl in his entire career as a WR (although he has another pro bowl and 1st team AP All Pro as a returner).
Just among his peers, Moss, Harrison, Owens, Bruce, and Holt would be unquestionably more deserving. Higher peaks, just as much longevity. Those are the "slam dunk, no brainer" picks. In addition, I'd say Carter and Brown are both clearly more deserving among retired players not yet in the hall. That makes 7 WRs ahead of Mason in the queue, and these are just the "there's not even any debate" guys on the list. I'd also say that Hines Ward, Jimmy Smith, Chad Ochocinco, and Rod Smith would almost certainly be ahead of Mason in the queue. I'd take Steve Smith over him, too, although that'll probably be a much more controversial selection. Also, Driver has one fewer 1,000 yard season than Mason so far, but Driver's also a year younger- Driver's as likely to get to 10 1,000 yard seasons as Mason is, and Driver would have 3 pro bowls to pair that with, so I'd take Driver over Mason, too. And that's not even counting the young guys- if Mason gets 2 more 1,000 yard seasons and then retires, then he won't be eligible for HoF consideration until 7 years from now. I'd imagine that by 7 years from now, multiple younger guys will have built up a much better case (Andre Johnson, Larry Fitzgerald, Reggie Wayne, Calvin Johnson, Roddy White, Miles Austin, etc).
Mason was a great receiver, but I think, no matter how the rest of his career plays out, the best thing anyone will be able to say about him by the time he's up for enshrinement is that he's the 12th best receiver not yet in the hall of fame. That's not much to base a candidacy on.