Stepney would honestly be a really great engine to own. He wasn't seen a whole lot, but is insanely popular among Thomas fans. One thing I like about Stepney that I feel really sticks out is his color. He's one of the only engines in the show with a color scheme so unique, and I believe it will surely stick out to many fans of Thomas, young or old. The other thing is the fact that his basis looks exactly like the one you'd see on the bluebell railway. As you said about the 75th anniversary approaching, having an older character which (in most cases) represents the bygone era of the models would be really neat. Older characters (such as the spiteful breakvan and Duck) seem to be a hit with fans who grew up with the show, so having Stepney in the line up would surely be great to see