I think MG is already on record saying the original AC theme will be used in the film... when and where has not been revealed, but I believe he's already said to expect the iconic theme to be included.
With that said, I don't think he'll compose a new main theme, a la Goldsmith for TMP, etc. I think the classic theme will be the main theme, with other original scoring thereafter.
I think it will be largely orchestral, but in a score like THE INCREDIBLES, he's clearly not averse to using more modern instruments at the same time.
More exciting (to me, at least), I think Giacchino has a very good ear for capturing and composing era-reminiscent scores. His work for THE INCREDIBLES had a good deal of 1960s jazz influences, per the film's 'nostalgic' tone and early James Bond flavor in action/mood.
And as I mentioned elsewhere, his overture for CLOVERFIELD also does an amazing job of capturing the sound and 'feel' of classic monster movie scores (GODZILLA, clearly). You don't have to recognize the references to enjoy the music, but if you do recognize them, his scores only get deeper to appreciate.
My guess is that his score might be quite reminiscent of the classic TREK style of score music -- not exact replications, of course, but I wouldn't put it past him and J.J. to throw in a bit of TOS "fight music" when appropriate.
Also, his use of female 'choral' vocals in his "Roar" Overture for CLOVERFIELD reminded me a lot of the TOS theme with the vocalist. Not in the melody, but how he incorporated the vocal with the orchestra -- again, highly reminiscent.
One of Giacchino's strengths is composing for the story at hand and the spirit of the film. He doesn't simply write underscore music, but music that "sounds like the film" your watching. Quite similar in effect to John Williams in that regard. I'm sure looking forward to his work for TREK!
MISSION:TREK's in-depth review of STAR TREK
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