I'm thinking the starting town is the one at the very bottom, with the red/yellow triangles. I just rewatched the very first XY-trailer, and it had the male character running through route 3. After this route you can see a town with buildings with green roofs. In this new trailer, we probably see the same town which also has buildings with green roofs (the one with the Roselia-fountain). And when looking at the map, a town with green roofs and a fountain is seen a little below Lumiose City and what looks like a bush labyrinth. Soo, you'd think route 3 is close to the starting city, and one option could be just up to the north of the town at the very bottom.
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