IME, the person's idea for the first product is the reason they start the micro ISV in the first place. I don't know of anyone who started an ISV, and then thought of a product.
Just to clarify ... I am not seeking to start a micro ISV without an idea. I am wondering if this approach is practical to pursue. If it is, there may be ideas/possibilities I have not thought of. When looking through SaaS-colored glasses, everything starts to look like a SaaS. :)