Phoenix is and will grow almost like a staircase. Due to mountains, but mostly reservations, this swath of land can/will be developed, but not the ones to the east or south of it. Or west and north of it. From Anthem in the north, you go south 30 miles, then 20 miles to the east, and then 10 miles south again.
The west side has some more room to grow, but it's all poor people and the retiree mecca of Sun City.