Just thinking out loud...
There's not been many consistently dominant goalies the last few years. Vokoun and Miller come close, but it's not like when you have Roy/Brodeur/Hasek all battling it out. So many goalies nowadays play such a similar style and are so technically sound that they rely on the team in front of them. I mean, Thomas led the league in 08-09 and this year in save percentage while Rask led the league last year with the same team.
IMO, he'd have to win at least two more Vezinas and have a few really excellent playoff runs, including at least one Cup and possibly a Conn Smythe. He'd have to be named first team All Star a few times. Then he gets in the discussion. The reason I say that is because the other individual awards, such as obviously the Calder and probably the Hart, are likely off the table.
If he had 4-5 years of dominance both in the regular and post season, we could talk about him being the best goalie of the late 2000's/early 2010's...but right now you could argue that Miller and Vokoun, maybe even Lundqvist are better goalies, just stuck behind worse teams. And I don't think any of those guys are HOF'ers.
EDIT: Also another reason I doubt he'll make it is that Rask is primed and ready to steal his spot, and I'd guess the Bruins would want a tandem at the least as early as next season, just so they have their goalie of the future winning and getting confidence rather than leaning on a 40-year-old. And that would reduce Thomas' chances to impress.