This is going to be a massive level, that's absolutely clear. Lots of varied places and therefore the need of pretty many background scores.
For any musician and/or composer who feels a bit intimidated to jump on the train, here's a few general tips for making music for this level:
General tips on using reverb:
A good way, both in game bgms and movie scores, is to work with subtle amounts of reverb to place the song where it belongs, namely not in the centre of focus. You want to avoid cheap linear and semi-linear reverbs as they sound horrible in almost every context. Spring and plate reverbs can be hard to put to good use for this, too, if not handled carefully. Aim for impulse reverbs or other physical modelling reverbs if you have CPU and economy for it. There are reverbs dedicated to put on the master channel in your DAW environment.
Depending on if the scene is outdoors/indoors, you might want to fiddle around a bit with the early reflections/reverb tail mix ratio. And remember, don't forget to be subtle when working on the master channel.
Making percussion, drums and other attention grabbing sounds have less impact (a few ways to handle it):
-first, see if you can reduce the amount of notes played.
-diminish the percussive sounds relevance by cutting them thinner with an equalizer
-if the base material is too punchy, and you don't want to alter it with eq, you can always flatten sounds a good bit with your favourite compressor by adjusting attack and release values (which is quite the opposite what you normally do with a compressor, if you are making, for example, rock music or house).
-once again, reverbs, in the right amount, puts drums on a distance.
Other tips:
-Don't overuse pan effects. In fact, don't aim for too much effects at all, as they possibly can steal attention from the game too much.
-Aim for a few chords, ambience or melody lines that aren't too messy, but still can be recognized upon hearing a second time.
-Sounds that stick out of the picture have a tendency to grab attention. This is background music, therefore, this is probably undesireable.
-effects which emulates spatial environments can be of good use as an addition, especially if all else fails for some reason but you still feel the music in itself is right.
EDIT: Feel free to come up with more tips and i put them here, or ask me if anything is unclear regarding this post.