In the meantime, I started a new game, One Man Armada (OMA), with a different physics engine, Box2D.Xna. My first impression, when compared to the Farseer Physics Engine, is that Box2D.Xna is much, much faster (it is optimized for the Xbox 360), but has less features. Farseer is close to releasing a new version, but right now I'd recommend Farseer for a PC game (or an Xbox game where you really need the more advanced features) and Box2D.Xna for an Xbox game where performance is essential.
All my games start with a basic gameplay idea that evolves according to whatever I think is working best as I playtest the game. :) I guess it's mostly the same for everyone.
OMA's basic idea is what would happen 9 months after Geometry Wars got drunk and had a foursome with Peggle, Breakout and Freespace. It's a dual-thumbstick shooter (with ships, not abstract stuff) where you shoot balls, like in Breakout, that bounce off ships, causing more damage with each bounce. So you must try to hit as many ships as possible with each shot while avoiding enemy fire.
Right now I'm working on a basic prototype and getting to grips with the physics engine. If the game is fun I'll follow up with it; if not I'll think of some other game. I did a logo anyway: