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I've kicked this about in my head also, because what geek wouldn't love to do it? I've come to the conclusion though that unless one is already a business and engineering genius with the luck of the gods, it's a ridiculous dream. With that said, a chap called (Sir) Martin Sweeting started Surrey Satellites in the 80s which went on to revolutionise the satellite industry and was sold partially to SpaceX, and then fully to ESA, so it is possible, but he was already a PhD-level scientist/engineer working in the field. Anyway here's a few ideas:

1. You could look at novel launch methods, i.e. centrifugal, rockoon, light-gas space-gun. On the latter Gerald Bull did reach space but needed a truly enormous gun

2. ...but imagine the know-how and resources required to develop a whole system like above that can get something to orbit. And quite apart from building the thing, how are you going to test it without become a lawyer to get the necessary approvals? So perhaps concentrate on one aspect of an example above that is transferrable to other industries. But even then to investigate materials that perform well at high temperatures, for example, it would help to already be a world-leading materials scientist, wouldn't it?

3. Fabrics. Looking at the latest NASA space suit I refuse to believe there isn't scope for improvement, and apart from that I suspect there'll be all kinds of novel fabric requirements. Again you might well find what you develop can be sold for earth-bound use

4. IP. Could you take an ARM-like approach and be merely an IP licenser? You could for example design modular launch systems and let others build them