I had a more detailed post written before and now it's been eaten by the ether, so here's the abridged version:
- an old article rumors Shimomura to be the sole composer; I doubt it
- I think FF15 will have multiple arrangers, maybe some additional composers
- past FF soundtracks are 4-disc affairs; Shimomura's longest is maybe KH2, which had multiple arrangements of Disney originals, and some tracks were handled by an orchestrator
- Shimomura hasn't done anything in recent years approaching the length of a typical FF soundtrack
- doubtful she's still working on XV, after all, Kingdom Hearts III is a thing
- with the change in director, more music is probably being requested
- recent trends, especially the XIII series, substantiates the expectation that multiple arrangers at least will be present
- or maybe it'll just be a half-size FF OST given the switch to action genre... after all, there's not a bit of music to be heard in the field, and so much of the music, thus far anyway, is performed live
The FFXV demo has the following music cues:
-main title theme "Somnus" (live piano + string ensemble)
-a little bit of the main FFXV theme (not Somnus... the OTHER one, the orchestral waltz with the prelude at the end)
-a short cue as Noctis wakes up and surveys the Duscae region... quite beautiful (live orchestra)
-main battle theme (live orchestra)
-main battle theme-night (remix of the main battle theme... seems to retain the live strings but adds live electric guitar, a louder piano part, and drum loops)
-boss-ish theme, same from a trailer from long ago (live orchestra)
-tension, enemy in sight, short loop (synth)
-fanfare, very short, no loop (synth)
-chocobo theme, folk-blues style (live harmonica + typical blues rock ensemble, in the game it's mixed to sound as if coming from a radio)
-camp theme, instrumental bossa nova style (live)
-somber theme at the end of the demo (live orchestra)
The tension theme doesn't sound like Shimomura really. The chocobo theme was probably arranged by someone else. Same with the fanfare and camp theme. Otherwise, typical Shimomura is on display, though I think her battle theme is quite nice. It's got the same rhythm she uses in all her battle themes, but here she attempts more interesting development.
Since the new director, Hajime Tabata, seems to favor Takeharu Ishimoto, I think he's a likely candidate to have taken over any extra music or arrangements. Mitsuto Suzuki possibly, as well. Mizuta is out as not only is he handling the music for the new FF iOS RPG by Takashi Tokita, but he is also likely working on the music for the final FFXI expansion.