Chuck Colson is apparently in gave condition.
Colson, as you probably know, had a famous prison conversion and has been doing good works (along with a fair amount of politics, I believe) since the 1970s.
But of course our concern here is with Watergate. It's worth pointing out that if there are any significant Watergate-related secrets that Richard Nixon took to the grave, Colson is probably the last person living who would have a good chance of knowing them. Collectively, Colson, Haldeman, and Ehrlichman presumably knew almost everything that Nixon knew, and the other two are gone. John Mitchell, too, could have told Nixon something that the others didn't know. Rose Mary Woods died in 2005. I can't imagine that Nixon revealed anything significant to his children. Or, for that matter, Pat Nixon, who died long ago anyway. I'd be shocked if Nixon told anyone who he started working with during his retirement anything we don't know.
I very much doubt that John Dean has any knowledge of what Nixon knew that he hasn't spilled long ago. G. Gordon Liddy, David Young, and Egil Krogh are still alive, and may know of some Plumbers activities that never became public, but it's not likely that they would know whether Nixon knew about them or not (and they almost certainly wouldn't know firsthand). I suppose it's vaguely possible that there's something that Henry Kissinger knows. Dwight Chapin? Seems unlikely.
What is left to know? The known unknown is prior knowledge of the Watergate break-in, and Gemstone generally. I'm also curious about the unknown unknowns. Could there have been significant additional Plumbers projects? Significant additional wrongdoing outside of the Plumbers framework?
The one thing that I'm confident we don't have a complete record of is the "Nixon said/ordered crazy things" category. I'm confident of it because tapes have been released fairly late in the day that added to it, and so it stands to reason that there was more of it that happened when the tapes weren't running. And Colson in particular is likely to have been around for plenty of that sort of thing. Of course, we have enough to make the context of Watergate perfectly clear -- Nixon may not have ordered the specific felonies involved in the Watergate break-in, but he frequently urged his staff to commit similar felonies for similar reasons. But more detail is always good.
Ah well; there's always the chance that some new documents or tapes or other evidence will turn up in the future.