Taking a page from everyone else, here is my list of themes and idea:
-Submerged
-Isometric
-Campaign Starter

Idea:
So I may be copping out by choosing Campaign starter as my last one, but my idea is to design an isometric underwater mine that would be the first dungeon a campaign could run. There will be some flooded rooms, with the mine starting on the shore but eventually going under a lake. A couple different floors and dead ends with some trapped miners that need to be rescued on the first adventure for your party.