So I have a few ideas. If this is too much, let me know and I can pare it down to my favorite.
Themes Related to the "Motor" in Motor City
Replaceable Parts
Internal Combustion
Man vs. Machine
Phrase Themes
Which way is up?
Are you thinking what I'm thinking?
Raise the stakes
You can also find a random theme generator based on previous Ludum Dare suggestions here: http://www.cowfacegames.com/handytools/ludum-dare-theme-generator