I'd certainly go for a Star Wars themed set or two. Here are my ideas for my dream sets (these ideas are freely given, Lionel or MTH...).
I'd like a pair of F3 sets with decorated PS-1 boxcars depicting various characters. In one set, the F3's could be painted black and red and the boxcars could depict Darth Vader, Darth Maul, Darth Tyranus (Count Dooku), and Darth Sidious (Emperor Palpatine) - a Sith set, as it were. The other set would be a Jedi set, painted in white (with blue, green and purple accents where appropriate) with boxcars depicting Yoda, Obiwan Kenobi, Mace Windu, and Luke Skywalker. Later, they could offer Anakin Skywalker as an add on car for both sets (good Anakin and pre-suit dark-side Anakin), and they could also offer a Qui-Gon Jinn as an add-on, as well.
They could also do some interesting freight cars, like a Death Star exploding boxcar...
Andy