There are 2 trains (sets) I would love to see offered:
1) From the movie Unstoppable, a GE AC4400CW AVWR 777, along with the EMD SD40-2 1206. I've seen some videos that feature custom painted versions of these.
2) The train from Back to the Future III along with a 1:48 scale Delorean complete with blinking LED lights. I'm not sure how they would make the smoke change from green to yellow to red, but either way they would sell a million of these sets!!!!!!