If we could get Tucson-Phoenix-Wickenburg going -- and the line is quite straight and fast from Phoenix to Wickenburg -- it would be a good start.
Corridor service up that line through Williams and Flagstaff might be viable even though slow, for Grand Canyon traffic alone...
The Prescott Line right-of-way seems completely intact from the north (east) all the way through Whipple. On the west, the line from Prescott through Iron Springs seems to have had a road or trail built on top of it. Around Wildwood Estates the right-of-way seems completely lost.
If the Ash Fork - Crookton line were rebuilt it would allow Amtrak to get rid of the crazy Williams Junction business and stop in Williams.
I've proposed upgrading the Arizona & California Railroad line from Wickenburg through Parker to Cadiz. Gain Phoenix and Wickenburg, lose Yuma, probably just as fast as the existing Sunset Limited route... more expensive than rebuilding the line west from Phoenix to Yuma, though.