I've only scored for a couple student projects, but it seems that music is added pretty late into development (also looking at how a lot of industry developers do it). (also Michael Giacchino had 4 weeks to make the music for Rogue One). I think the non-music sound guys are working on it from the beginning though, cause there's soooo many assets to be made, and it seems like a different beast.
You can make tracks and stuff for the concept art, but keep in mind that this early it will be subject to change. Some of the tips from Berklee Online class I took on Game Audio Production are to examine the game assets and stuff, write down the mood, feel, etc, then surround yourself with things it makes you think of, other inspirations/sources (games you've played, movies, etc). Make an audio mockup to some video footage to give yourself some goals/direction for your music.
This is my general impression, but perhaps someone from industry can answer better!!