If they're serious about expanding to Houston and Atlanta (and by all accounts they are). then they need two more cities. to balance the Conferences/divisions. 17 teams each in the Eastern/Western conferences doesn't really work for nice clean divisions. If on the the other hand you add four teams, you'd have 18 in each conference and could do two divisions of nine teams each or three divisions of six (which, actually, I kind of like).
And with a four team expansion, yes a return to Arizona makes complete sense. I hope for Arizona's sake they get a management team that actually knows what they're doing this time.
Atlanta (East)
Houston (West)
Arizona (West)
Real question is, who would be the 4th team? Should be a team that would slot naturally into the Eastern conference. Another western team would push Chicago or Nashville east and result in them being the only team in that conference not in the Eastern Time zone -- something which they would really hate.