Brainstorming is important to your preplanning. Here you want to sit down, maybe even with others, and write down anything that comes to mind regarding your venture- whether it seems relevant or not. And don't think about what you're writing down, just write and worry about thinking about it later.
The results of your brainstorming will (hopefully) lead you to random dimensions of your venture and add areas of consideration, possibly even leading to a niche aspect. At the very least it will help you add to your business plan and show that you have fleshed out your venture. It will also project to potential investors that you are prepared, especially when acknowledging obstacles and risk by asserting practical solutions to overcome these issues.
Tip: Don't be afraid of, or avoid, acknowledging risk. Risk IS business and the greater the risk, generally the greater the reward. Addressing risk serves two critical purposes in planning to launch a business; 1) Recognizing it beforehand allows you to determine if your venture is truly feasible and will save you a lot of blood, sweat and tears if acknowledged before launch, or outcome 2) By knowing it's there, if you come up with a practical solution to overcome it, you may launch your business with leverage over the competition- leading to business bliss- success!