Remember, restrictions breed creativity.
This line here!!! I think that this is the reason that games like DND flourish, especially with homemade campaigns. The DM is limited in what they can do to provide experience for the players, so they have to use creativity and personality, which always makes for a fun campaign.
a. Do you want to give players an experience? This approach is about using theme as the core idea for your game. Will your game be about an epic fantasy adventure? Or how about exploring and colonizing the galaxy?
I think that this is the most important facet of tabletop games, at least for me. I have played plenty of random table top games with my roommate in the past, and I think the defining feature of the games that determined if I enjoyed them or not was if the game provided a good player experience in the way of concept and immersion.