From a commercial standpoint, Jethro Tull seems like the next progressive rock band to get in.
The Moody Blues like The Monkees, lack momentum. If something came up to raise their visibility (sadly, a member dying might be the only way, but that didn't seem to help The Monkees even when Davy Jones died, and we'll see if 2/3 of ELP dying makes any difference over the next year or 2).
Sure, King Crimson would warrant it given both Critics and even Hipsters praise them, but even if they get on the ballot, the majority of the voters (not the fans necessarily) have never heard of them or heard 1 of their songs sans for maybe through a movie (Buffalo'66 or Children of Men) or Kanye West's sampling of 21st Century Schizoid Man.
On Dio, he didn't get in with Sabbath? I suppose he could get in as a pick for "Musical Excellence" like Nile Rodgers just did.
Maiden, Motorhead and Priest seem to be Metal bands who are long overdue. I imagine Megadeth would soon follow.