I never saw Proven play, so I can't comment on him.
From the players who I saw play, I think Peter Sterling or Brett Kenny should be inducted as the next immortals.
Brett Kenny was 5/8 for Australia on the 1982 Kangaroo tour keeping vice captain and immortal Wally Lewis out of the team. On the 1986 Kangaroo tour, Wally Lewis was captain and 5/8. Brett Kenny was that good a player that they had to find a spot for him in the Australian team, so they played him centre, thus keeping Mal Meninga on the bench. Gene Miles was Kenny's centre partner, so Miles was also rated better than Meninga on that tour. Meninga gets touted as a future immortal quite often. In the 1985 Challenge Cup Final of Wigan vs Hull, Kenny had a blinder and was named man of the match. In that game the old English commentator said about Kenny. You will never see a better stand off (5/8) in this country and that's if you've ever seen one. When Kenny played 5/8 for NSW against Lewis I read that NSW won 8 games to QLD 4. Even Lewis said that Kenny was his greatest opponent. I think Kenny scored doubles in all of Parramatta's premiership wins of 81, 82 and 83.
I rate Kenny the best 5/8 I have seen and obviously better than the immortal Wally Lewis, who I have a very high opinion of too.
Peter Sterling, Andrew Johns and Thurston are the 3 best halfbacks I have seen. Sterling was the best general I have seen play the game. Thurston is a definite future immortal and Sterling should be named one too.
Sterling and Kenny are the best pair of halves I have seen. It was a shame that NSW selectors have been so pathetic for so long. Sterling and Kenny played very few origin games together at half and 5/8.