Just as my experience with PEI was the culmination of applying an interest in databases with my experience in finance, your best ideas will likely come from your area of expertise. This is where you have the greatest chance of executing successfully. From within your area of expertise, your idea will either be a product/service/tool you wish you had, or it will be a more efficient way/process of doing a particular job.
Because of this, you are apt to have more ideas and better concepts as you expose yourself to a myriad of circumstances and environments that teach you trade skills. If you are in a position for 18-24 months, you should readily find opportunities for improvement, either as an employee or even within your own company.
Good ideas are simply a matter of efficiently solving a problem you know exists in the marketplace. Just be sure that it’s a sufficiently large market such that others are experiencing the same problems and the pain point is great enough that they are willing to spend money on it. If you save time in a process (thereby reducing labor costs or turnaround time) or if you make something cost-effective or if you make something of higher quality for the same price compared to the existing product, you’re on your way to a successful idea.