Since the goal is to maximize cash flow you have evaluate ideas based on the factors that will result in biggest return to you in the shortest amount of time.

First thing you have to evaluation is how much revenue does the idea have the potential to generate. This has a variety of sub categories. Like does it fill a need in the marketplace, are people willing to pay for it and how much are they willing to pay.

The next factor should be cost to deliver, both in terms of direct costs to produce the idea or service and second is how much cost will it require to manage the process. This second goes hand in hand with scalability. It's great to have a product or service that generates a high margin but if requires too much time to manage it is probably not a viable business idea.

There are other considerations as well, such as legal and logistics but I think the first two items will get you started in the direction you wan to go.