There have been several attempts to set up photographer-owned combines, and I don't think any worked out.
My first stock agency was Tony Stone, in London. He became the biggest home-grown one and he himself was an ex-photographer and therefore very aware of photographic realities.
He used to supply lots of stock images for those multiple calendars that printers used to produce, where a company only able to afford, say, fifty units could buy that small quantity, the only thing really being their own was their company overprint. Many printing giants ran these services. One, probably the biggest in the UK at the time, was Bemrose in Derby, who printed a couple of bespokes for me. Unbeknownst to me, the chap who handled my work there referred me to Stone and I received an invitation to visit in London and was subsequently asked to join, which I did.
The split with Stone was 50-50 on all sales, and though sales abroad via sub-agencies was supposedly split differently, I found no reason to complain. But I ran into what, for me, was a big flaw: most of my good sales turned out to be in Fance or other foreign parts. On the basis of thinking my style was more suited to the European than British market, I requested that my submissions be edited in Stone's French offices by French eyes. This request was denied. I then approached Image Bank in Paris, and was told that my submissions would have to be edited in Spain, by then my country of residence if not domicile. (Don't even attempt to understand - I never have). Then, Stone became Getty's first huge buy, by which time I was gone.
I would love to find a new outlet that allowed me to make money from setting up and shooting shoots again!
So, my feeling about setting up new agencies is that the difficulties are even greater today, and the time/application needed to run one would quickly swamp any indiviual.