With DBPro you don't have to start with 2D programming. You can go right to 3D, it's not much harder.
What's your favorite type of game? RPG? RTS? Strategy? Puzzle? Try to make a watered down version of something you really like. If you don't like the game genre, it is sometimes hard to get excited about your project.
Other bit of advice is for the first simple game, make it complete. Give it several screens (title, options, high score, etc.) as you will learn a lot just by doing that.

A 3D marble platformer using Newton physics.