Am I overstepping with a proposition to further my career?

I work for a small to medium company and have worked my way up the ladder over 15 years to a managerial position in a specific department. That department comprises of solely me and I take on a large workload that is constantly expanding.

As far as I can see career wise it is as far as I can go without changing roles altogether, not that I want to or can as there is no one to take my position due to my varied skill set that have put me in this position. My choices seem to be to push for something more or leave for a larger company with more scope for progression, however, I believe in this company and want it to grow. I have been here a long time and we are a close knit bunch directors included.

I have a proposition in mind to make this side of the business a subsidiary company (they already have another) with the same branding and for me to be made director. I believe should benefit them tax wise and I would take on whole responsibility for this area of business. I just need to figure if this idea would benefit the company enough to get them to go for it. The main benefit I can see would be that it would prevent me from being forced to look elsewhere.

I’m not the most business minded person but am constantly striving for more and no one is going to give it to me so I need to try and made shit happen!

Am I completely overstepping and insane?

If not what would be a good way to propose such a thing?