I think composers ultimately have to make a compromise between choosing to go fully ambient or thematic.
on the one hand, ambient is highly immersive but also forgettable..
and on the other, themes are distinct and have identity, but since they're "created and not natural" pull away from immersion.
it thus lies to the composer to create something that is memorable but also immersive at the same time. if we look at area music in particular..
- Uematsu in XIV was almost entirely thematic, and the apparent disregard for proper instrumentation pulls you out of the immersive experience really quickly no matter how good the melody of the track is. Take On Windy Meadows for example. It sounds great, but the image it conjures in your mind is probably nothing like where the track appears in game.
edit: also, wrt instrumentation, contrast Conflagration which sounds like some groovy disco tune, with the ethnic feel of Pitfire, and you'll see how thats an issue with the old OST.
- Hamauzu in XIII is very ambient. The immersion is great tho because the use of instrumentation is consistent and the area themes sound pretty ethereal, just like the settings. However, when he does use themes it takes over the entire track. I love The Gapra Whitewood for the ambient feel.
- Sakimoto in XII leaned more to the ambient side with arpeggios, harps and trumpet build ups but did create very short thematic ditties for key elements like the theme of the game in general and the empire.. which were repeatedly used in many tracks to create that kind of familiarity and association. Like in Mosphoran Highwaste. That said, his ambient is still very emotionally charged (which can be a bit overenthusiastic at times.. not suited for MMOs for sure)
edit: that said, theres no battle music in open areas in XII so the bgm kinda needs to give you that kind of atmosphere of a struggle or some kind of building tension..
- Mizuta in XI has always been more thematic than ambient., but does use instrumentation to play up the latter to great effect. His stuff is pretty good for MMOs because they're typically emotion neutral save for the mood that the area is meant to convey. Illusions in the Mist is probably his best example of this.
His strengths are in very measured use of melody, the ability to create themes that appear neutral-sounding and his incredibly varied use of instruments.
Soken right now, based on what we've heard thus far seems to be more ambient. I'd think similar to Sakimoto, especially with incorporating recurring ditties to create familiarity (the tune at the start of the Black Shrouds alpha video is also found, slightly muted, in the Gridania video when they enter Old Gridania). The instrumentation style is probably closer to Hamauzu in terms of being a lot more subtle..