Regarding writing an E&E game that would sell, here is a little rant based on my observations over the years:
(warning: rantish)
1. You need a distinct and appealing art style. This means hiring an artist with a vision, getting a professional to do proofing and layout. The game must look original and separate - here are a few recommended picks (focusing on just a few examples that I own - please don't hold it against me if you find your favorite item missing):
- Broken Tales
- Blade Runner
- Orbital Blues
- White Hack 3 (the art style is literally a basic LaTeX template, and yet it is so unbelievable easy on your eyes that it makes for a very cozy read)
- Necrobiotic
- Arc Doom
- Troika | Electric Bastionland (can't believe I failed to include this)
2. The system must be serviceable. It may be super simple, it may be more elaborate, but the underlying concept must be simple, at least on paper. Only big brands can afford to be elaborate with impunity (Level Up/A5E).
2.1 Simple systems that are too simple or too casual will die. The system must be able to produce engagement that rewards its mastery. I know that some people people disagree with Monte Cook who said this originally, however... if you want your players to read some of your books, you should give them stuff to engage with the game on several levels.
2.2 Prior to any publishing campaign do your best to get other people to use your system. Chances are that there are things obvious to you, and not necessarily to others.
2.3 Too many indie games recently produced are all about a system, but not enough about a setting. Not enough setting content means that GMs will have to rely on tropes and stereotypes, and most GMs will at some point give up the game because of this.
3. The setting material is not about a number of new races, artifacts or monsters. It's about what the role of protagonists is, culture differences, mysteries, politics, economics and plots.
3.1 I cannot stress enough that the protagonists role must be addressed - are they adventurers? If so, what kind of work are expected the adventurers are to do? What do the NPCs are doing meanwhile?
3.2 If you are producing a derivative E&E setting, do remember that there is a glut of generic online content. Do try not to reproduce it.