Devin Booker obviously needs a more aggressive, competitive Chris Paul type to provide the leadership he can't provide. Jimmy Butler was the only available such personality I knew of, and he's gone. i have no idea who else of this type could be acquired in the offseason.
It's not a problem of basketball ability that caused the failure of this team. It's the personalities. There was no one strong enough to keep Kevin Durant's moody, self-absorbed and un-productively laid back personality under control, the way Michael Jordan and Phil Jackson kept Dennis Rodman under control sufficiently to make him an integral championship contributor. Certainly, Devin Booker couldn't influence Durant: Booker is a weaker personality; and besides, he's much younger in a basketball context. Everyone thinks that the exact opposite happened: Durant influenced Booker, and not positively.
Although, I acknowledge, has anyone ever helpfully influenced Durant? It doesn't appear so. Durant has walked away from every previous team he's played for, often in a huff because things somehow didn't go his way. The only thing I give him credit for is that he doesn't appear to be mad at the Suns over talk of trading him. At least, not enough that he clearly wants to leave the Suns the way he left other teams.