However, established systems shouldn't be overlooked. They have two major advantages.
The first and commonly acknowledged advantage is that established systems take the GM from "zero" to "ready" in a minimum amount of time. The GM doesn't have to build the rules, the world, the balance, the combat tables, the monsters, the NPCs, the maps - all that stuff can be pulled from content already in the system, although he's free to create it on his own if he wants to.
However, a system has a second advantage which is not commonly talked about: system content.
A world like D&D contains a huge amount of rules and content. The players can spend a lot of time trying to figure out the best build, debating which monsters are stronger, whether wood elves or moon elves are better, talking about whether the ancient war of Kalvyos actually exterminated the shadow gnomes, and so on.
They can do this without the GM - either on their own just perusing the books, or in tandem with other players. The GM shouldn't dismiss this kibitzing just because it has no direct effect on the game. It's player engagement: it makes the players want to play more, want to immerse themselves more.
Even in a weak ruleset, the system content can make up for it. See: every Star Wars game ever.
Part of the popularity of D&D is specifically because it has lumpy, irregular rules. Players can argue over minutiae like whether armor class should go up or down. Perfectly logical and abstract rules are what most indie developers aim for: "these are the simple, unified rules that you use for everything!" They're actually shooting themselves in the foot! Lumpy rules and content give the players traction.
System content is so valuable for raising player engagement that I think it's important to factor it in to your own systems. Of course, generating enough content is a challenge. Also, the kind of content to generate is important.
I think the content that matters most is rule-linked lists, preferably mixed with a lot of flavor text.
By "rule-linked list", I mean any variety or list of things which interact with the rules in some way. For example, each class has a list of level-up capabilities and restrictions. There's a list of dozens of kinds of guns, each with its own subtly different statistics. There's a list of planets each space nation inhabits. There's a narration about the last war, full of places and combatants and rule-legal results...
Basically, "rule-linked list" just means "don't stuff your guidebooks full of backstory and worldbuilding that is supposed to be interesting on its own: Make sure it ties into the rules and mechanics."
A common trap is to just create huge stretches of samey text. So, when building content, make sure to switch it up. Common elements to combine:
Narratives ("the elves struck back by...")
Explanations ("Elves live for thousands of years...")
Stats ("Elf: +2 dexterity, -2 strength")
Templates ("Generic elf warrior:")
Guidance ("It's usually best for the GM to present elves like...")
Images of characters/monsters/closeups ("this is an elf')
Images of the setting ("this is an elf village")
Technical-oriented images ("here are common elf weapons...")
So make sure to switch it up. Don't worry about balancing the types: you just want to make sure the player never spends more than five minutes without switching gears, or it can be boring.
But how can you build such a large amount of content?
Well, dedication is the classic way. Just keep making more until the game feels ready.
It's not very realistic, though. Another way is to make it a community project, and allow the early audience to give you content of their own creation. This'll probably work if you have an early audience.
Yet another way is to steal content from another, established setting. This is problematic because you need to build a lot of "glue" to translate that content into your system, but on the other hand, translating more content is something which players often love to do.
The last way I can think of is heuristically. In many situations it'll be possible to automatically create many kinds of details - for example, you can automatically generate thousands of pages of maps covered in nations, and even write up the summary of each nation automatically. You could also create an image system which could create schematics for any kind of gun (with many random visual variations), and combined with a gun-randomizer, create a thousand different kinds of guns.
The problem with such algorithms is that they create very repetitive, bland content. One option is to massage them manually, creating flavor text and leaving out the most boring ones. Another option in this internet age is to package the algorithm.
Why would you need to print out a blank character sheet? Link to a web site that lets you build the character. This other web site generates guns. This one generates nations. Put a small selection in your book, but then challenge the players to use the randomizers and generators on-line.
Anyway, however you approach it, I think you need to consider system content when developing a new system. A system that is too smooth and sleek doesn't give players any traction when they want to make the move from "newbie" to "fan".