I'd suggest doing three things to narrow down the field:
1.Try to sketch the business model for the startup idea
http://businessmodelgeneration.com/canvas
2.Get good at doing quick and dirty market sizing of opportunities
3.Check if you are passionate enough about the idea to potentially spend the next 5-8 years of your life in making it successful
Based on a collective evaluation of the above three, you might be able to zero in on a few ideas to investigate further (e.g. building a prototype, customer development, etc.)
more doing - implement and test the concept
Focus on sales, even if the product doesn't exist yet.
Do you enjoy selling this product?
Do you understand who the market is?
Do people want to buy it?
Do they want to buy it even if you haven't created it yet?
Will they give you money for it now even if it won't be ready for a few months?
Set some targets for yourself. For example, commit to calling at least 100 prospective customers and trying to sell the product before you give up on the idea, and don't move forward until at least 6 people have agreed to buy it (or paid in advance).
1. Make a list of all your business ideas.
2. Eliminate the ideas that you cannot execute on within six months in your current status. It doesn't matter why not -- whether you have enough money, time, or talent to do it. Contrary to popular belief, starting a company does not start with the product -- it starts with what YOU can do. You may have the best product in the world, but if you are incapable of selling it or finding people who can, your company will go nowhere.
3. Of those that remain, you must do your research. By that I mean, you first must google the hell out of it. Most likely, someone else is already doing something similar, or it was already tried and a company failed at it. Find out why it failed. If another company is doing it, dig deeper to find out how they are doing, and what markets they are currently. Also, research the bios of all their team members and compare them against your own team. If you are falling short on expertise compared with them, it's probably not a good idea to directly challenge them.
4. Do more research. Talk to a CFO who is in the industry and ask him or her what the likely profits are for each.
5. Choose the idea that will generate the quickest amount of profits in the shortest period of time.
Notice I said nothing about passion. That is because passion is way overrated. I've seen companies run right into the ground because the founders were so passionate about it they refused to alter critical elements.
Some of the best companies I have seen the founder had very little passion for. Rather, they were in an industry, and they saw an opportunity to make money where no one else did, so they jumped on it and made it into a fast growing company.
It is NOT about passion -- rather it is about finding a product that the market needs and you can sell and scale quickly. If you can't do that, all the passion the world isn't going to save you.
