There is no box. Franchises deliver an easy mold for uninspired developers to copy from, while the avant-garde of game design continues to reach for unconventional thinking. The problem with each of those options is that you're either designing inside a box of what works or fighting to be different from what works—sometimes to the point of trying too hard. Indie game publishers need you to design games as if there was no standard to conform with. As if there wasn't anything to fight against. Take a look at what game publishers need from you as an independent designer, and remember to allow your imagination to soar.
Dream First, Package Later
Whether you're looking for an idea to spin to a publisher or a complete product, ask yourself one question: "Is this the dream?"
Did you compromise with your idea? Was that compromise because of technical limitations, or did you change in order to be sellable? Do you have a big story that you think needs cutting, or are you unsure of how to deliver the game the right way? As you ask these questions, make sure that you write your concerns alongside the original dream.
Don't destroy the game idea, even if you think it isn't possible. Even if you think you are (or actually become) the greatest game developer of your age, you'll never know what small team or accidental discovery might creep its way into the world. If you cut away parts of an idea and lose or forget the idea, you could be jeopardizing a much bigger project.
Build Towards Breaking The Scope
Although the gaming world is flooded with remakes and sequels, there's a place for continuation. Deliver the entire idea as well as your deliverable idea with cuts in mind, but keep the original dream alive for a potential re-release. If your sales-ready, publisher-approved version is successful, you can use it to power a revisit. It isn't against the rules, and fans will be thankful for a new work closer to the original idea.
It's important to maintain a scope for certain projects, but this shouldn't end in completely maiming your idea permanently. If you have gameplay elements that won't work with current systems or content that is too graphic for audiences, simply think of a supplement for the main idea that works with another genre.
Use your limited resources to make something that can be paired with a better idea yet to come. You're not compromising, you're giving players a whiteboard of your brainstorming and future plans in the form of playable, complete games.
Speak with an indie games publisher to discuss your plans for development and ways to work towards idea that may not have traditional release options—or to discover a release option catered to your plans.