Essentially, mainstream camera making is not something that can "feed" a large company anymore - that's obvious.
I don't see Nikon jumping into a new market at this point - they don't have the money nor leadership quality.
So IMO they can either accept the role of small, high-end camera maker (somewhere between Fuji and Leica) or they can try to diversify by merger or selling into a larger company (but keeping the brand).
Aside from cameras and optics, the only thing they have expertise in is semiconductor lithography. They're years behind ASML, but good enough for less cutting edge solutions. Maybe they could focus on that... who knows...
Anyway, as of 2020 Nikon is still a major manufacturer, with skilled workforce and a very good brand. And they were very successful and profitable even few years back - unlike Olympus, who was a business failure for nearly 2 decades (albeit making very good products).