Will Phoenix and Flagstaff connect soon and become one giant city? ?

No, they will not connect soon.  They are abut 145 miles apart and, too, 1-17 goes through some pretty rugged country.  Not too many suburbanites would want to live there.  Although Phoenix grows, week by week, day by day, maybe even minute by minute, it would take many decades before it could reach that far north.  

Flagstaff is still pretty far away. But I was surprised to find out that Scottsdale and Tempe were basically suburbs of Phoenix. I always thought they were separate cities. 

Soon? No. Not a chance.