In terms of accomplishments and WAR he's pretty much a lock to get in. Mauer is already 12th all time as a catcher in terms of JAWS (Total Career WAR and their 7 year WAR peak averaged). That should improve through the life of his contract. Beyond that, he's won three batting titles, an MVP, a few gold gloves, silver sluggers, etc. He's also been the best catcher of the last 10 years. He should be one of the few guys that basically everyone can agree on, whether you prefer statistics, award hardware, or by the yardstick over whether he was dominant at his position and regarded as the best. He might not get in immediately, but he'll get in quickly.