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