This is useful, thank you.
Starting a separate company to go after some of these market opportunities is a good idea; and one that I've thought about as well. I should have made it more clear that I wasn't trying to negotiate for them to give me their company, but to negotiate significant ownership interest in it.
Since I've left, they've asked me to come back and offered significantly higher salary, shown interest in licensing software from me that would take care of some of their business problems, and "would consider any other option" that I brought to them in order to come back and take the reins again.
I do have a track record of generating results for them, and their growth has completely stalled since I left.
Approaching this as a separate company in which I'd ask them to invest, I don't see much difference in the end result. I'd still want to hold significant ownership interest in the new company, I'd still be asking them for money to fund the venture, the risk appears to be similar to me. I am certain that I can significantly improve the value of the company because I already have paying customers lined up for this new functionality.
I agree that even asking for 50% after 10 years is ludicrous, but I want to consider ludicrous options.
The proposition that I'm making to them is to consider the value of holding 100% of the company as things go today, versus holding half (10%? 20%?) of the company at whatever value can be created between now and then.
I suppose a better way of approaching the question would be this: A client has been actively recruiting me to return and take over management of their software company. How can I maximise the outcome / what is possible via aggressive negotiation?
When I left their employ and they became a consulting client again, I was able to negotiate a pretty ridiculous seeming consulting arrangement with them and they have been very happy with it thus far. But it left me feeling like I wasn't aggressive enough in negotiations.