Beyond Starters and obvious early route niches, here's a simple shorthand for working up to a dex. Decide on some basic themes or aesthetic, and then populate...
>Town
>Neighboring Route(s)
>Associated Dungeons
Rinse and repeat

Don't forget to account for your GymLeaders' teams when populating routes. And of course Victory Road and your E4 and Champ as well. I wouldn't worry about sticking to 150, but maybe keep each route under 6 unique evolutionary lines to reduce bulk.