If you do a Deluxe book with errata and maybe some of the cooler stuff from supplements folded into the core book, and maybe some new art, I will happily buy a copy.
Well, to be honest, if they did a "Deluxe Player's Rulebook" with all the background, career information, equipment and rules from the career books to date, I think it would do very well... it wouldn't need to have all the information, indeed that would be unwise, since people wouldn't buy the books, but just the careers alone would make it worthwhile - the rules and most of the specific background could be referred to the career books, with just the basics being brought into the main book.
A "Big Bumper Book of Referee Fun" could be good too - but I kinda like having separate books when doing different GM tasks, although books that actually stay open would be a move I'd appreciate (which is why I'm slowly moving towards PDFs - I can keep those just one alt-tab away instead of trying to keep a softback book open.
My favourite would be to rewrite High Guard, redoing the ship design rules, perhaps bringing them more in line with the new vehicle rules if people like them and with notes explaining in more detail the accepted tolerances and practices when designing deckplans, as well as various sizes of staterooms and common areas. For example, a warship would probably be happy with 4-square staterooms that are double-occupied - indeed, you may even get 4 people to a 6-square area, if you have double bunks on either side of a central "corridor", most likely with a shared fresher at the end of the block - most likely used for enlisted crewmen where space is at a premium (that's similar to the bunkspace on a WWII sub). Compare that to a larger, luxury stateroom on a private yacht and you can see how the tonnage may vary, let alone the allowance needed on the deckplans.
Splitting the design rules off into their own rulebook would also be a valid (in my opinion) option, also... allowing them to be looked at in more depth. Perhaps, then, High Guard could discuss various things like probable fleet strengths and the chain of command - things that are only really covered in Sector Fleet, but would apply to non-3I settings also.