Not to revive an older post but pretty much everything is missing.
The opening, which is also used when Jack dies
The wreck music [2 1/2 Miles Down]
When the safe is recovered [To the Keldysh]
Brock sees the portrait for the first time [Rose Revealed]
"Yes...I would like to see my drawing"
The film version of Southampton (in 5/4)
When Jack and Fabrizio run to catch the ship [A Lucky Hand]
The film version of Leaving Port
Take Her To Sea, Mr. Murdoch (mostly the same as on the album, but the ending is different, although fortunately this is used in the end credits)
Jack first sees Rose [First Sighting]
Rose's Suicide, mostly kind of eerie drones used in the earlier stages of the sinking and, of course, Rose's suicide attempt
Trapped on D-Deck, which is used throughout the sinking
....and (like my heart) it goes on and on. There are so many differences between the album and film versions, either they're completely different or mixed differently (like the enhanced "claps" in "Unable to Stay, Unwilling to Leave" and the removed choir in "Rose" [another differently mixed version of it is also used when Rose's hand comes against the glass in the car]). Also not included are Gaelic Storm's contributions (the Irish music) and a few of the classical pieces (most were released but "Oh, You Beautiful Doll" - which is used prominently in several scenes, like the scene with Titanic sailing into the sunset before Jack attends the dinner - and a very-low-in-the-mix "Come Josephine In My Flying Machine").
As far as I know there are no plans to release anything, but I've got a petition to release the complete score that will hopefully raise some eyebrows (it has gotten the attention of some higher-level people). I'm trying everything I can to get additional cues released by the companies who hold the rights - right now there doesn't seem to be a plan but in speaking with some execs they actually didn't know that not everything had been released (unfortunately the 4 CD anniversary release led to a prevailing misconception that everything has been released already). But after 20 years ... it's about time, I'd say.