Other than morals, there's also laziness as a possible impeding factor.
so yeah, feel free to start early! Or finish late! Or just repost a game that you made years ago that's already a tangled mess!
The only frowned-upon rule breaking is not putting everything into a single class/struct/whatever.
Regarding themes: there aren't any, really. But here's a few if you want ideas:
- Dogs
- Pancakes
- World peace
- Plants
- Power plants
- Rubber chickens
- Wigs