How to choose which business idea to postpone
An idea can wait with a clear reason and a condition for returning to it.

Imagine you already deliver advisory work to clients and want to develop a course. You are interested in teaching, and you also have an idea for a more specialised client service. Both could use your experience. Your current assignments leave room to explore one of them.
Part of choosing is deciding what you will leave alone for now. Give the postponed idea enough attention to explain why it is waiting and what would make you return to it. That lets you examine a particular decision when circumstances change, instead of starting the whole comparison again.
Find what you want the idea to change
Ask what attracts you to each possibility. You might want more time on a subject you enjoy, a different relationship with clients or a way of delivering that asks less of your own time. Describe the change you want in your working life.
In the hypothetical course example, you could be drawn to helping a group learn together. Recording material and selling access to it would ask for a different kind of work. Before deciding to postpone the course, clarify which version you were considering and which part you want to do.
Keep that reason with the idea. If you return to it later, you can ask whether it still offers the change that interested you. You may discover that a different arrangement in your existing work would serve the same purpose.
Name what would have to make room
Use the commitments you already have as the starting point. Which work have you promised, and what part of your week would preparation for the new idea use? Include time outside work that you want to protect.

Finishing a particular assignment might release the time you need. Taking on another similar assignment might use it again. If you intend to begin the course afterwards, examine that next client decision as well. Waiting for work to become less busy leaves open what you will do when more work arrives.
You can choose to keep the current work. You can also decide that you want to change the business enough to make room for the idea. Describe the responsibility you are willing to finish or change, and what that would require. A decision to postpone becomes clearer when you can name the work taking priority.
Give the postponed idea a condition for returning
Write down what would make it worth another look. In this example, completing a current assignment could be a condition. A request from a particular kind of customer might be another. Choose a circumstance that relates to the reason the idea is waiting.

Consider what happens if the condition is met. If the assignment finishes, are you willing to leave some of that capacity available? If a customer asks about the course, what would you need to understand before agreeing to prepare it? The condition should bring you back to a decision you can make.
A date can be useful when you want to reconsider your own priorities. At that point, look at what has changed and whether the reason for postponing still applies. You can continue to leave the idea aside. An automatic start date would commit you before that conversation with yourself has happened.
Separate waiting from a question you can answer
Sometimes the idea is waiting because you do not yet know something that affects the choice. If you are unsure what a team wants to learn, a conversation with them may help you decide whether the course deserves further work. That is a smaller commitment than developing the material.
Be precise about the question. What would the team need to be able to do afterwards? Are they asking for help with that task, or have you inferred it from a general interest in the subject? Keep their actual request separate from the offer you would like to create.
You can then decide whether finding out belongs in the work you are choosing now. If it does, give it a limited place alongside your existing responsibilities. If it does not, record the unanswered question with the postponed idea so you know what remains open.
Keep a record you can return to
A short note is enough: what you wanted from the idea, why it is waiting and the circumstance or question you want to return to. Leave the detailed preparation until you have decided to give it time.
Bring that note and examples of your current work to business coaching if you want help examining the choice. We can look at what you want to keep doing personally and which next step you want to take. The first conversation stands on its own; you decide afterwards whether to continue.