With the interest in Arizona railroads on the rise with Lionel's announcement of the Grand Canyon Railway set, I thought I might make my own suggestion for a custom run of another Arizona RR. Magma Arizona RR #10 seems like a great candidate as it is a Baldwin DRS-6-6-1500, which the MTH Railking AS-616 is a good model of. I'm sure there are differences, but seeing as there were only ever 83 real world DRS-6-6-1500's made, I doubt we will ever see a model of it in O gauge. Also, the locomotive is preserved and can still be viewed at the Arizona Railway Museum in Chandler (an outskirt of Phoenix) which makes the prospect even more appealing!
With MTH making the announcement that they are doing a run of the AS-616, now is the time!
Image Attribution: user:Jot Powers, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
More Info about Magma Arizona RR:
More Info about the Baldwin DRS-6-6-1500: