Venus Patrol

I haven't played all missions yet, but I don't think you need a lot. Perhaps a short Soviet campaign, or a very simple mission generator a la Chuck Yeager's Air Combat would be enough?

The missions are hugely re-playable as they are. The top-notch physics mean that the aerial ballet always plays in a slightly different way.

In terms of commercial viability Proteus is a good example:
http://store.steampowered.com/app/219680/
It's a slow-paced exploration game with a very strong style, but being relatively short didn't stop it from being released in Steam to general acclaim.

Just a thought. Thanks again!