For my first released game, Space Bunny Zita, I got inspiration from the game Round 42. I loved the use of a modified CGA text mode, so I based my aesthetic on that. The HUD and Game Over screen pretty much are lifted from Round 42.
To be honest, the way I went about developing my game from there was probably one of the worst ways of going about it. I made mockups of what I planned for the key elements of the game. Initially I had three portions planned, hence why the folder I kept assets in is called TT, for "Triple Threat". The three parts were to be a top-down shooter like Robotron, a driving segment like the 8-bit Ghostbusters game, and an artillery game like Paratrooper on DOS or Airborne! on Mac. The idea was that you had to drive around to random parts of a city under invasion and either destroy enemy strongholds or defend the skies. The player would be on a Crazy Taxi-esque timer and completing tasks would reward you with more time and score.
I got as far making the shooter and driving segments, with the Paratrooper clone never making it that far. As I was testing to see if things worked, the driving segments felt painfully boring compared to the rapid pace of the shooting. It didn't help that the shooting segment was taking up far more of the C2 free 100 events than I was expecting. So I decided to axe the driving and focus solely on the shooting. I think like half of the prototypes I sent to my friend still have remains of that portion before I got around to removing it entirely. I fumbled with concepts and the controls as I basically taught myself how to use C2. I didn't plan on having music, opting for an ambiance to give that feeling of playing on an old IBM PC. One of my friends who wrote music asked me what I planned for music, and when I described what I would use he came to me a day or two later with a WIP track, the one that would be played in-game. Even near the end of development I was making pretty big changes. Eventually I put the game out there and from there I've been tweaking it to refine it some more as I learn more about C2.
TL;DR - I basically just fumbled around until I had something solid.
For my next project I've put a lot more thought into it and not just throwing stuff at the wall to see what sticks. I do plan on going back to Zita when I get C2 Personal and truly "complete" it, adding just the little things that I couldn't do with C2 Free.